Flink: Deprecate 1.20 Flink support

[talatuyarer]

Deprecate Flink 1.20 support in next release. This change removes all code related to Flink 1.20, as version 1.11 is now available. This ensures we focus our efforts on newer, supported Flink versions.

[nssalian]

Worth sending a dev list note too about this.

[pvary]

We usually create a single big PR where there are 4 commits:

• Rename the current top version to the new one - this helps us keep the history (2.1->2.2)
• Copy that version to the original one - this restores the current version (2.2->2.1)
• Drop the old version (1.20)
• Fix the build, so the new version compiling - we can review the required changes independently

We merge these commit without squashing them, but at the same time, so the history is clear.

This is a big change, and every other Flink PRs need to pause a bit. Since there is quite a bit of development happening, we might want to coordinate on the dev list.