roachtest: increase admission control mixed version timeout

Recently, this test started running with more workload steps, each of
which takes 5min, so it's been timing out consistently. This commit
increases the test timeout to 3 hours.

Fixes: #143331

Release note: None

Author: miraradeva

pkg/cmd/roachtest/tests/admission_control_elastic_mixed_version.go

@@ -37,7 +37,7 @@ func registerElasticWorkloadMixedVersion(r registry.Registry) {
 	r.Add(registry.TestSpec{
 		Name:             "admission-control/elastic-workload/mixed-version",
 		Owner:            registry.OwnerKV,
-		Timeout:          1 * time.Hour,
+		Timeout:          3 * time.Hour,
 		Benchmark:        true,
 		CompatibleClouds: registry.OnlyGCE,
 		Suites:           registry.Suites(registry.MixedVersion, registry.Nightly),