Issue #614 - update package imports

[nikosbosse]

Description

This PR closes #614.

This PR

• removes the lifecycle package from imports as we aren't using it anymore (at least for now)
• adds a version requirement to testthat as suggested in #614

On that note, we should maybe configure our R CMD check CI to fail when encountering a note. I think it currently let's Notes pass.

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [x] I have added a news item linked to this PR.
• [ ] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

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

Project coverage is 95.27%. Comparing base (7df87d7) to head (4e15b71).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #723 +/- ## ======================================= Coverage 95.27% 95.27% ======================================= Files 21 21 Lines 1630 1630 ======================================= Hits 1553 1553 Misses 77 77 ```

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