Pull Request validation tests

Fix 3.0 and drop 2.6

Failed in 17m 38s
:ruby: 2.6.expeditor/buildkite/run_linux_tests.sh
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Ran in 1m 2s
:ruby: 3.0.expeditor/buildkite/run_linux_tests.sh
Waited 1s
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Total Job Run Time: 21m 27s