chore: Update services and docs to Python 3.12 as default

[matthewfeickert]

Description

• Update to using Python 3.12 in linting and validation tools.
• Update Binder runtime Python to Python 3.12.
• Update Dockerfile base image to python:3.12-slim-bullseye.
• Update the docs to advocate using Python 3.12 for development.

Checklist Before Requesting Reviewer

• [x] Tests are passing
• [x] "WIP" removed from the title of the pull request
• [x] Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• [x] Summarize commit messages into a comprehensive review of the PR

* Update to using Python 3.12 in linting and validation tools.
* Update Binder runtime Python to Python 3.12.
* Update Dockerfile base image to python:3.12-slim-bullseye.
* Update the docs to advocate using Python 3.12 for development.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.21%. Comparing base (dd4a243) to head (2886bea).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2461 +/- ## ======================================= Coverage 98.21% 98.21% ======================================= Files 69 69 Lines 4541 4541 Branches 804 804 ======================================= Hits 4460 4460 Misses 48 48 Partials 33 33 ``` | [Flag](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | Coverage Δ | | |---|---|---| | [contrib](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `97.79% <ø> (ø)` | | | [doctest](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `98.08% <ø> (ø)` | | | [unittests-3.10](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.23% <ø> (ø)` | | | [unittests-3.11](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.23% <ø> (ø)` | | | [unittests-3.12](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.23% <ø> (ø)` | | | [unittests-3.8](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.25% <ø> (ø)` | | | [unittests-3.9](https://app.codecov.io/gh/scikit-hep/pyhf/pull/2461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | `96.27% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep#carryforward-flags-in-the-pull-request-comment) to find out more.

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[matthewfeickert]

@meeseeksdev backport to release/v0.7.x

[lumberbot-app[bot]]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout release/v0.7.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

   git cherry-pick -x -m1 db67210ec76007db1a5b6818644fe721662d5249

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #2461: chore: Update services and docs to Python 3.12 as default'

4. Push to a named branch:

git push YOURFORK release/v0.7.x:auto-backport-of-pr-2461-on-release/v0.7.x

5. Create a PR against branch release/v0.7.x, I would have named this PR:

"Backport PR #2461 on branch release/v0.7.x (chore: Update services and docs to Python 3.12 as default)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.