Add Java and Android SDK upgrade instructions

[bendk]

I think this captures the process correctly, but please double check me. The reason I'm adding this is that we're going to be adding another place to edit in #6366 so I figured it could use some documenting.

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • [ ] This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• [ ] Quality: This PR builds and tests run cleanly
  • Note:
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  • If this pull request includes a breaking change, consider cutting a new release after merging.
• [ ] Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Changelog: This PR includes a changelog entry in CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• [ ] Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due diligence applied in selecting them.

Branch builds: add [firefox-android: branch-name] to the PR title.

[bendk]

I also added Java upgrade instructions while I was at it.

[codecov-commenter]

Codecov Report

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

Project coverage is 52.70%. Comparing base (3760c22) to head (0d67131).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6367 +/- ## ======================================= Coverage 52.70% 52.70% ======================================= Files 125 125 Lines 12824 12824 ======================================= Hits 6759 6759 Misses 6065 6065 ```

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