minor symfony#12501 Adjust upgrade file rendering & backport missing …

…notes (stloyd)

This PR was merged into the 2.3 branch.

Discussion
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Adjust upgrade file rendering & backport missing notes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
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d9c2e6d Adjust upgrade file rendering