Drop support for PHP lower than 8.2

[stof]

Dropping support for old PHP versions simplifies our maintenance work, by unlocking a bunch of improvements (which will be done in separate PRs):

• updating Psalm to a maintained versions
• updating to PHPUnit 11
• investigating the usage of newer PHP features (typed properties, readonly properties, etc...) when doable without BC breaks.

Note that this also fixes the correctness of the implementation of #1882, which relies on union type that are not compatible with PHP 7 (not detected by the CI as we don't yet generate code needing such shape, but there is an in-progress PR that will use it once updated).

Closes #1961

[stof]

Remaining TODO; figuring out the proper way to document this in the changelog files so that we release it as a semver-minor, not as a semver-patch.

[stof]

The changes in the psalm baseline correspond to different kinds:

• updating the psalm version on the root node (apparently, it's been a while since the last time we did an automated generation of the baseline as it was with an older version of Psalm)
• reordering files in the baseline (manual edition of the baseline did not preserve alphabetical sorting)
• removing some errors in Core that are no longer reported thanks to more precise type declarations in psr/cache and symfony/cache, thanks to using newer versions now that Psalm does not run on PHP 7 anymore.
• new errors reported in the Monolog integration due to running Psalm using Monolog 3.x instead of 2.x (fixing them will requiring dropping support for Monolog 2.x, to migrate to modern Monolog APIs)

[stof]

@jderusse @Nyholm could one of you remove all PHP versions lower than 8.2 from the list of required CI checks ? I don't have admin access on the repo to do it myself.

[jderusse]

@jderusse @Nyholm could one of you remove all PHP versions lower than 8.2 from the list of required CI checks ? I don't have admin access on the repo to do it myself.

done.

Feel free to merge the PR when you think is ready.
Thank you for this contribution and pushing this change.