Update Slim to 3.13.0 for PHP 8.1+ support

Slim 3.13.0 was released in April 2026, with support for PHP 8.1+,
despite the earlier statement by the Slim project team that 3.x was
end-of-life and would not be updated.

Consequently, the workarounds that were implemented to support PHP 8.x
with Slim 3.12.5 are no longer needed and can be reverted.

• Bumps slim/slim from 3.12.5 to 3.13.0
• Reverts the workarounds (795ff02e5500008f055589038ae0404ed575537a,
  d5a30a0cb15e617749a1feb0ef46dbe782b522b2)

Fixes 0037288, PR https://github.com/mantisbt/mantisbt/pull/2242