Replace references to GdUnit3 with GdUnit4

[esainane]

Why

The default C# template still had using GdUnit3. It's easy to change the default template (thank you!) from project settings, but this should be a little nicer on everyone.

What

This changes the template to have using GdUnit4. This also changes error messages in GdUnitTcp{Client,Server}.gd to be attributed to GdUnit4 instead of GdUnit3.

Some references still remain, left untouched here:

• One in FUNDING.yml, which appears to be an account name unlikely to have changed, and I wouldn't feel comfortable touching this one anyway.
• One each in bbcode_example.txt and markdown_example.txt. As these are examples taken from that era, I think these are fine to leave?
• gdUnit3-examples and all of its references. Given there are res:// links to here, I'm not sure what migration would look like. I'm at least not confident in this being as simple a set of changes as the ones provided in this PR.

[MikeSchulze]

Thanks for findings and fixes 👍

• FUNDING.yml,

Yes, this is the "old" account name and should not be changed.

One each in bbcode_example.txt and markdown_example.txt.

These are only test resources, and it does not matter whether they contain GdUnit3 or GdUnit4.

gdUnit3-examples

The examples are currently deactivated, they still need to be migrated to Godot4