Use `server_version_num` instead of parsing version()

[Algunenano]

Some PostgreSQL forks (e.g. EnterpriseDB) modify the version() output so matching PostgreSQL ([0-9\.]+) doesn't work. This affects postgis_extension_remove_objects and _postgis_pgsql_version functions.

Using SELECT current_setting('server_version_num') should be equivalent and clearer.

[Raul Marin]

In 17178:

Use server_version_num instead of parsing version()

References #4296

[Raul Marin]

In 17179:

Use server_version_num instead of parsing version()

References #4296

[Raul Marin]

In 17180:

Use server_version_num instead of parsing version()

References #4296

[Raul Marin]

In 17181:

Use server_version_num instead of parsing version()

Closes #4296
Closes ​https://github.com/postgis/postgis/pull/363

[Raul Marin]

In 17205:

Partially revert r17179

It changed the behaviour of _postgis_pgsql_version
Now uses the appropiate 5-6 digit version from server_version_num

References #4296
References #4303

[Raul Marin]

In 17206:

Partially revert r17180

It changed the behaviour of _postgis_pgsql_version
Now uses the appropiate 5-6 digit version from server_version_num

References #4296
References #4303

[Raul Marin]

In 17207:

Partially revert r17181

It changed the behaviour of _postgis_pgsql_version
Now uses the appropiate 5-6 digit version from server_version_num
References #4296
References #4303