roachtest: disable 23.1 -> 23.2 testing for follower reads

After introducing shared process testing of follower reads, the tests
follower-reads/mixed-version/* have been flaky. This commit starts
testing from 23.2 to make sure that any other failures are caught and
triaged correctly.

This test is flaky when upgrading from v23.1 to v23.2 for follower reads
in shared-process deployments. There were a number of changes to tenant
health checks since then which appear to have addressed this issue.

Epic: none
Fixes: cockroachdb#133000
Fixes: cockroachdb#132999
Fixes: cockroachdb#129546
Fixes: cockroachdb#129167

Release note: None

pkg/cmd/roachtest/tests/follower_reads.go

@@ -1000,6 +1000,12 @@ func runFollowerReadsMixedVersionGlobalTableTest(
 		// Use a longer upgrade timeout to give the migrations enough time to finish
 		// considering the cross-region latency.
 		mixedversion.UpgradeTimeout(60*time.Minute),
+
+		// This test is flaky when upgrading from v23.1 to v23.2 for follower
+		// reads in shared-process deployments. There were a number of changes
+		// to tenant health checks since then which appear to have addressed
+		// this issue.
+		mixedversion.MinimumSupportedVersion("v23.2.0"),
 	)
 }