Auto generate changelog and readme entries (simplified version)

[wjrosa]

Complex version #3537

Changes proposed in this Pull Request:

This PR changes how we update changelog.txt and readme.txt entries by making it more automated. New entries are generated and committed when the option Automatically create a changelog entry from the details below. is checked (added to the PR template), and the required information is provided.

By checking This Pull Request does not require a changelog entry. (Comment required below), the changelog entry is not created automatically.

Testing instructions

Code review should be enough. You can also draft another PR with a branch created from this one (dev/auto-generate-changelog-and-readme-entries2) and test the new command and see the new PR description template.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)