Fix and update Changelog builder

[sambles]

Issue Description

Switching to squashed merges for release is causing problems for the changelog builder script, the tags don't quite line up.

Example, running this returns no PR's

update-changelog.py build-changelog \
 /home/runner/work/OasisLMF/OasisLMF/update-changelog.py build-changelog \
   --repo OasisLMF \
   --from-tag 1.28.3 \
   --to-tag 1.28.4 \
   --github-token *** \
   --local-repo-path ./ \
   --output-path ./CHANGELOG.rst \
   --apply-milestone

However, selecting the tag before correctly finds all PR's between 1.28.3 and 1.28.4,

 update-changelog.py build-changelog \
 /home/runner/work/OasisLMF/OasisLMF/update-changelog.py build-changelog \
   --repo OasisLMF \
   --from-tag 1.28.2 \
   --to-tag 1.28.4 \
   --github-token *** \
   --local-repo-path ./ \
   --output-path ./CHANGELOG.rst \
   --apply-milestone

Also, need to filter for PR's starting with Release <version> and exclude these from the notes

[sambles]

This problem comes from squashing the release branches when merging back to stable. After the merge the tag becomes orphaned

Solution for now is to not squash and merge release branches. alternative is to re-work release pipeline to tag the merged commit after squashing