Drop Flink 1.15 and 1.16 support

[kennknowles]

Fixes #32838 and #32835

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[kennknowles]

Looks like the last success for the Typescript Tests was last year, if I am reading this right: https://github.com/apache/beam/actions/workflows/typescript_tests.yml?query=branch%3Amaster+is%3Asuccess

[kennknowles]

LGTM, I created kennknowles#11 to remove a few old version references.

Nice! Love the PR-to-a-PR-branch maneuver. Thank you.