https://gerrit.wikimedia.org/r/c/operations/puppet/+/1242483 is upgrading Java on releases hosts from version 17 to 21.
Bookworm does not provide Java 21 natively, it's a locally maintained component.
The upgrade here is to precede Gerrit being updated to Java 21 which is required by the latest LTS version of Gerrit.
As Moritz points out on the patch above this is why it would also be good if releases hosts were upgraded to trixie which natively provides Java 21.