Make Geo work with PostgreSQL 12

What does this Merge Request do and why?

For PostgreSQL 12, instead of creating recovery.conf, the script writes the settings to postgresql.conf (except the standby_mode), and creates standby.signal file. This is a simpler version of !1560 (closed).

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Related issues

#968 (closed)