Use constants in the names of the two fields that are currently readable or writable in file versioning

[bdovaz]

Is your feature request related to a problem? Please describe.

Currently the use of author or label fields is referenced without constants:

server/apps/files_versions/lib/Listener/VersionAuthorListener.php

Line 51 in 9e0e537

$this->versionManager->setMetadataValue($node, $node->getMTime(), 'author', $author);

server/apps/files_versions/lib/Sabre/Plugin.php

Line 79 in 9e0e537

$propFind->handle(self::VERSION_LABEL, fn () => $node->getMetadataValue('label'));

server/apps/files_versions/lib/Sabre/Plugin.php

Line 80 in 9e0e537

$propFind->handle(self::VERSION_AUTHOR, fn () => $node->getMetadataValue('author'));

server/apps/files_versions/lib/Sabre/Plugin.php

Line 89 in 9e0e537

$propPatch->handle(self::VERSION_LABEL, fn (string $label) => $node->setMetadataValue('label', $label));

Describe the solution you'd like

In my app I set by code through IVersionManager the label metadata value and I would like the reference to be more secure through that constant.

I can create a PR if you approve.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[provokateurin]

@bdovaz can you create a PR and link it here? This seems quite easy to implement and review.

[bdovaz]

@provokateurin done! #53832