use jdk 21

[matzew]

currently we have older java 17: https://github.com/knative-extensions/eventing-integrations/blob/main/.github/workflows/build.yml#L32

The other java bits in knative are already on OpenJDK-21:
https://github.com/knative-extensions/eventing-kafka-broker/blob/main/.github/workflows/knative-java-test.yaml#L31

[matzew]

@christophd we could change all in source, and also touch the archetype.

Not sure we want to "re generate" exisitings?

[pierDipi]

And also we would need to change the compiler version

eventing-integrations/pom.xml

Line 16 in 6934237

<maven.compiler.release>17</maven.compiler.release>