Release process: automate squashing changelog entries for Beta releases

In #21143, we introduced a new `squash` command that one can use to squash changelog entries in the changelog file. This can be useful for example to take changelog entries from all Alpha releases in a given month, into a single Beta changelog entry at the end of the month:

`cd projects/plugins/jetpack && vendor/bin/changelogger squash`

It would be nice if this were included in the `tools/plugin-changelog-to-readme.sh` script, so squashing happens automatically for Beta releases.

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Internal references:

- p9dueE-3dC-p2
- p9dueE-3S6-p2

Start with tools/plugin-changelog-to-readme.sh and the vendor/bin/changelogger squash command described in the issue. Trace how the script handles Beta releases, then verify that Beta processing invokes the squash behavior while preserving the existing changelog-to-README flow.

Written by the indexing model from the issue text.