prepare release 2.8.1

[stephprince]

Prepare for release of PyNWB 2.8.1

Before merging:

• [x] Make sure all PRs to be included in this release have been merged to dev.
• [x] Major and minor releases: Update package versions in requirements.txt, requirements-dev.txt, requirements-doc.txt, requirements-opt.txt, and environment-ros3.yml to the latest versions, and update dependency ranges in pyproject.toml and minimums in requirements-min.txt as needed. Run pip install pur && pur -r requirements-dev.txt -r requirements.txt -r requirements-opt.txt and manually update environment-ros3.yml.
• [x] Check legal file dates and information in Legal.txt, license.txt, README.rst, docs/source/conf.py, and any other locations as needed
• [x] Update pyproject.toml as needed
• [x] Update README.rst as needed
• [x] Update src/pynwb/nwb-schema submodule as needed. Check the version number and commit SHA manually. Make sure we are using the latest release and not the latest commit on the main branch.
• [x] Update changelog (set release date) in CHANGELOG.md and any other docs as needed
• [x] Run tests locally including gallery, validation, and streaming tests, and inspect all warnings and outputs (python test.py -v -p -i -b -w -r > out.txt 2>&1). Try to remove all warnings.
• [x] Test docs locally and inspect all warnings and outputs cd docs; make clean && make html
• [ ] After pushing this branch to GitHub, manually trigger the "Run all tests" GitHub Actions workflow on this branch by going to https://github.com/NeurodataWithoutBorders/pynwb/actions/workflows/run_all_tests.yml, selecting "Run workflow" on the right, selecting this branch, and clicking "Run workflow". Make sure all tests pass.
• [ ] Check that the readthedocs build for this PR succeeds (see the PR check)

After merging:

1. Create release by following steps in docs/source/make_a_release.rst or use alias git pypi-release [tag] if set up
2. After the CI bot creates the new release (wait ~10 min), update the release notes on the GitHub releases page with the changelog
3. Check that the readthedocs "stable" build runs and succeeds
4. Either monitor conda-forge/pynwb-feedstock for the regro-cf-autotick-bot bot to create a PR updating the version of HDMF to the latest PyPI release, usually within 24 hours of release, or manually create a PR updating recipe/meta.yaml with the latest version number and SHA256 retrieved from PyPI > PyNWB > Download Files > View hashes for the .tar.gz file. Re-render and update dependencies as needed.

[codecov[bot]]

Codecov Report

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

Project coverage is 91.91%. Comparing base (c99c0c5) to head (fc78cf2). Report is 18 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1927   +/-   ##
=======================================
  Coverage   91.91%   91.91%           
=======================================
  Files          27       27           
  Lines        2648     2648           
  Branches      691      691           
=======================================
  Hits         2434     2434           
  Misses        141      141           
  Partials       73       73           

Flag	Coverage Δ
integration	72.80% <ø> (ø)
unit	83.98% <ø> (ø)

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