pytest: Add `system-postgres` DB provider and allow pyln-testing to run on older CLN versions too

[cdecker]

This tackles two problems:

• By using a system-wide installed postgres we can cut down on the overhead of spinning the DB up and down for the tests. More importantly this allows us to run the tests as root, since we no longer run into postgres complaining about not wanting to run as root. This is for example the case if we run in CI or in docker, where the files are often owned by root, and having to switch to another user can result in permission issues.
• The recent v23.11 release of pyln-testing broke backwards compatibility with prior CLN versions because it just bluntly assumes --developer is a valid CLI option. It was introduced in v23.11 and therefore causes all prior versions to just abort().

[cdecker]

Fixed a minor issue when parsing non-release version.