ci: add backend release notes to the release PR

[MilesCranmerBot]

PySR's release notes only said "update backend to v2.7.0", so 2.7.0 shipped without saying what changed in SymbolicRegression.jl. Backend sections had been added by hand to the release-please PR, but release-please rebuilds that PR on every push to master, which wipes manual edits.

A new job runs after release-please on every push. If the release PR's top release contains an update backend to vX entry, it replaces that entry with a section like

### Backend: SymbolicRegression.jl 2.7.0 → 2.7.1

PySR now uses SymbolicRegression.jl 2.7.1 (#1430).

#### SymbolicRegression.jl 2.7.1
<that release's notes>

with one subsection per backend release since the previous PySR tag. It edits both CHANGELOG.md on the release branch and the PR body, and does nothing when the section is already there, so it is safe to rerun.

Tested locally on the current 2.7.1 release PR with the #1430 bump applied: the output matches the example above, and a second run leaves both files unchanged. actionlint passes on the workflow.

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-15T22:26:10.202105Z	bba7551	PR opened

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