ci: skip oversized flake-check selections (#28293)

Skip the targeted Go flake check when a change selects more than 100
tests, and update `whichtests` to the version that supports this limit.

Large selections usually come from broad or mechanical changes rather
than a narrowly introduced flaky behavior. Repeating hundreds of tests
substantially increases runtime and runner resource usage while
providing less focused signal; regular CI still exercises the affected
test suite across its normal jobs and repetitions.

.github/workflows/flake-go.yaml

@@ -59,6 +59,7 @@ jobs:
             --repo-root . \
             --github-actions \
             --coalesce \
+            --max-selected-tests 100 \
             --out-matrix "$RUNNER_TEMP/flake-matrix.json"
 
       - name: Set up Terraform