fix: Use DI for Setup class and move away from deprecated methods

[come-nc]

Summary

Extracted from #40633 as it made more sense to clean up OC\Setup completely.
Migrate from deprecated methods to OCP\Server::get and DI.

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)

[Altahrim]

Nit:

Suggested change

	if ($installPath === null) {
	$this->isInstanceReadyForUpdates = false;
	} else {
	$this->isInstanceReadyForUpdates = true;
	$this->isInstanceReadyForUpdates = $installPath !== null;