sql: investigate whether session revival should be made available to UA clusters

**Describe the problem**

We currently gate session revival both on a non-public cluster setting and on whether we are in a secondary tenant. UA clusters will always be running in secondary tenant, making this secondary check less effective.

```
pkg/sql/conn_executor_exec.go:2754:79: AllowSessionRevival.Get(&ex.server.cfg.Settings.SV) && !ex.server.cfg.Codec.ForSystemTenant(),
```

```
pkg/sql/session_revival_token.go:43:75: AllowSessionRevival.Get(&p.ExecCfg().Settings.SV) && !p.ExecCfg().Codec.ForSystemTenant(),
```

```
pkg/sql/session_revival_token.go:79:77: if !AllowSessionRevival.Get(&p.ExecCfg().Settings.SV) || p.ExecCfg().Codec.ForSystemTenant() {
```

This seems relatively minor to me since the tenant setting itself is SystemVisible, so users would have to go out of their way to enable this non-public setting.

Jira issue: CRDB-34662

Epic CRDB-61004