Update `LDAP_USER_FILTER` env option to use new placeholder format

[ssddanbrown]

Update LDAP_USER_FILTER env option to use {user} in addition to ${user}.
The existing ${user} technically can be considered as a nested variable by dotenv. Most cases it's fine unless a user variable is defined but best to move away from this.
Doing something similar for #4732, so best to align these now.
Keep support for old format for forwards compatibility though.
Don't think it will conflict with existing cases, but maybe should still alert about this?

Doc Updates

• Update existing ldap docs with new format.
• Update advisory to suggest change and potential compatibility?

[ssddanbrown]

Closed via 6b68196