sql: All session settings to enable disable an option should end with `_enabled`

All cluster settings to toggle a thing currently end with `*.enabled`. However, no such convention is enforced for session settings. This has resulted in a mixed bag of session settings:
For example:
```
enable_auto_rehoming
index_recommendations_enabled
```
https://www.cockroachlabs.com/docs/stable/set-vars.html

We should add an alias for all session settings in the form of `enable_*` to `*_enabled` and prevent new session settings from being added in the form of `enable_*`.

Jira issue: CRDB-22537

Epic CRDB-60948