[Docs] Update Flink version references on the Flink runner page

[Eliaaazzz]

Updates stale Flink version references on the Flink runner page.

• The dependency example and the docker run commands use Flink 1.20 instead of 1.18. Flink 1.17 and 1.18 support was dropped in [Feature Request]: Add support for Flink 2.1.3 #38947.
• The pre-built Job Service image list drops 1.16, 1.17 and 1.18 and lists 1.19 and 1.20, and now also records that Flink 2 and later images share a single apache/beam_flink_job_server repository with the Beam and Flink versions in the tag, for example 2.75.0-flink2.0.
• The local cluster link points at the current Flink docs location instead of the retired ci.apache.org path.

The contributor guide that was originally part of this PR now lives in #39580, so the two can be reviewed independently.

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[Abacn]

For Flink 2+, the naming changed to, e.g. https://hub.docker.com/layers/apache/beam_flink_job_server/2.75.0-flink2.0

[Eliaaazzz]

Good catch, thank you. Added a line recording that Flink 2 and later share the apache/beam_flink_job_server repository with the Beam and Flink versions in the tag, using 2.75.0-flink2.0 as the example.

I also split the contributor guide out of this PR into #39580, so this one is just the version reference updates.

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[Abacn]

Thanks!