Upgrade frontend to Java 21

[ioannakok]

We should upgrade to Java 21. We are currently in Play 2.9 and soon will be in Play 3.0 after this gets merged:

• #26755

The official Play recommendation is to be in either 17 or 21. Java 11 support will be dropped in an upcoming Play release.

See also:

• https://docs.google.com/document/d/1ZR-YnaXCT5_gLVmTCeGs0mWd3KPaAozPjQK8uUzHZ9w/edit
• https://chat.google.com/room/AAAAag0I08g/Nvinl3aN_cU/
• https://chat.google.com/room/AAAAag0I08g/3fGxEDwXpjg/