Apply correct defaults for username / password

[DrRataplan]

Fixes #213

Defaults were already documented at https://github.com/eXist-db/xst?tab=readme-ov-file#with-environment-variables

[line-o]

That's @DrRataplan for your contribution. I am just unsure what the best default is here:

• why use guest/guest? A lot of requests do not need leveraged permissions and will now fail when using the admin account on instances that most likely will have changed the password (eg. xst info).
• why use admin/(empty)? It's convenient when working with test instances.

[DrRataplan]

I'm also unsure. I feel something for having leveled permissions: attempt using a guest user if we only need guest-like access, and admin if we require more privileges.
But this increases complexity.

Based on the comments in the file I touched and the README, admin/<empty> was intended to be the default. This used to be the actual behaviour, but that was removed in 508c75f. It's just that the comment and the README was not updated. This caused my (and @JoernT's) confusion.

Do we wanna go left or right?

[line-o]

@DrRataplan I would like to update the readme with the actual defaults

[line-o]

maybe together with #240

[DrRataplan]

I am up for that, especially combined with the dotenv changes: I will then just make a dotenv in the folder that houses all my projects!

[line-o]

Please change the readme to reflect the correct defaults instead.