[FEAT]: Update `python-semantic-release` usage (including templates) to take advantage of enhancements added in `python-semantic-release==9.10.0`

[nfelt14]

Description

python-semantic-release version 9.10.0 added a lot of new features to make changelog updates simpler. This should allow simplifying the actions/workflows in this repository to no longer need to copy over the previous changelog file contents.

Additional Information

See the information on the https://python-semantic-release.readthedocs.io/en/stable/concepts/changelog_templates.html page about customizing the changelog and the available variables/filters for the template.

This change would probably necessitate a v2 release, since it would break backwards compatibility in the find_unreleased_changelog_items action.

[nfelt14]

Relates to #163

[nfelt14]

Relates to #248

[nfelt14]

Relates to #499