See cockroachdb#52094 (review).

We don't currently track the bootstrap version of each secondary tenant.
For this to be meaningful, we'd need to record the binary version of the
SQL gateway that processed the crdb_internal.create_tenant function which
created the tenant, as this is what dictates the MetadataSchema that was
in effect when the secondary tenant was constructed. This binary version
very well may differ from the cluster-wide bootstrap version at which the
system tenant was bootstrapped.

Since we don't record this version anywhere, we do the next-best thing
and pass a lower-bound on the bootstrap version. We know that no tenants
could have been created before the start of the v20.2 dev cycle, so we
pass VersionStart20_2. bootstrapVersion is only used to avoid performing
superfluous but necessarily idempotent SQL migrations, so at worst, we're
doing more work than strictly necessary during the first time that the
migrations are run.

Now that we don't query BootstrapVersionKey, we don't need to have it
in the allowlists in the tenantAuth policy for Batch and RangeLookup
RPCs.