update changelog for 5.1.0 release

## Description regtests jwst : https://github.com/spacetelescope/RegressionTests/actions/runs/19139125132 romancal : https://github.com/spacetelescope/RegressionTests/actions/runs/19139131036 ## Tasks - [ ] [run `pre-commit` on your machine](https://pre-commit.com/#quick-start) - [ ] run `pytest` on your machine - [ ] Does this PR add new features and / or change user-facing code / API? (if not, label with `no-changelog-entry-needed`) - [ ] write news fragment(s) in `changes/`: `echo "changed something" > changes/<PR#>.<changetype>.rst` (see below for change types) - [ ] update relevant docstrings and / or `docs/` page - [ ] for any new features, add unit tests <details><summary>news fragment change types...</summary> - ``changes/<PR#>.feature.rst``: new feature - ``changes/<PR#>.bugfix.rst``: bug fix - ``changes/<PR#>.doc.rst``: documentation change - ``changes/<PR#>.removal.rst``: deprecation or removal of public API - ``changes/<PR#>.general.rst``: infrastructure or miscellaneous change </details>