fix(UserMountCache): Add back unique index for oc_mounts and use normal insert

[provokateurin]

Replaces #56929

With the unique index back there will be no more duplicate mounts, without modifying the transaction isolation level.
Also to avoid any future issues with this method, I went ahead and removed all usage (it was deprecated in 15.0.0 already).

Oracle will likely fail on the truncate for the same still unknown reason as in #54014.

[provokateurin]

Please review again. I only intend to backport the first commit, as the other ones are just improvements that are not strictly necessary to fix the bug. All tests should be passing now.

[SystemKeeper]

1. OCA\Files\Tests\Command\DeleteOrphanedFilesTest::testClearFiles
   TypeError: OCA\Files\Tests\Command\DeleteOrphanedFilesTest::getMountsCount(): Return value must be of type int, string returned

PHPUnit failures still seems related

[provokateurin]

Yes they are also, the files_external ones even though I don't understand why yet.

[provokateurin]

Hm sharding is stuck and an earlier run was canceled (by phpunit) due to timeout. I suspect this is related to the changes.

[provokateurin]

/backport to stable32

[provokateurin]

/backport to stable31

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31
# Create the new backport branch
git checkout -b backport/56933/stable31
# Cherry pick the change from the commit sha1 of the change against the default branch# This might cause conflicts, resolve them
git cherry-pick 341fd348
# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/56933/stable31

Error: Failed to check for changes with origin/stable31: No changes found in backport branch

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