chore: Fix update-psalm-baseline to use correct php version depending on branch

[come-nc]

Summary

The workflow currently always use 8.1, it should use the same PHP version as the psalm run on the target branch.
Adding the PHP version to psalm.xml so that the update-baseline workflow is aware of it. Backports will need to adapt the PHP version to the one used in their psalm workflow (the lowest PHP version supported usually).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[nickvergessen]

Should add it like in apps, e.g. https://github.com/nextcloud/spreed/blob/main/psalm.xml#L8
So it locally also always runs with the expected php version

[come-nc]

Should add it like in apps, e.g. https://github.com/nextcloud/spreed/blob/main/psalm.xml#L8 So it locally also always runs with the expected php version

Ok, trying this way then, putting 8.1 in master and will change to 8.0 in the backports.

[come-nc]

/backport to stable29

[nickvergessen]

Backporting is not going to work, 29 and lower need 8.0 at least.
So let's save 2 CI cycles by backporting it after 29 was fixed...