Server: Add --serverinfo ISO country code support

[hoffie]

Short description of changes
With this change, --serverinfo gains support two-letter ISO country codes such as "de", "gb", "nl", "fr". Numeric Qt5-style codes keep working as expected.

To simplify user-side debugging, the --serverinfo parsing now contains logging for the final values (or a warning in case of parsing problems).

CHANGELOG: Server: Added support for ISO country codes (de, gb, nl, ...) in --serverinfo.

Context: Fixes an issue?
Fixes: #2362
Related: #2829 (comment)

Does this change need documentation? What needs to be documented and how?

As this is a (tiny) new feature, docs would be useful, but not mandatory. This is not a breaking change. Delaying the docs update would be no problem.

Status of this Pull Request
Ready

What is missing until this pull request can be merged?

• Server: Fix --serverinfo country code misinterpretation on Qt6 #2829 must be merged first

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[hoffie]

Marking it as Draft to prevent accidential merges before #2829 is merged. Feel free to un-mask afterwards.
As to whether this should go into 3.9.1 or not: In theory, it's a (tiny) small feature and would therefore rather belong to 3.10.0. If you (mainly @pljones and @ann0see) deem it ready to merge, feel free to include it in 3.9.1. Otherwise, please move the milestone.

[pljones]

I'm hoping to get this in to 3.9.1 - mostly because I'd love to get away from the numbers in the server info :). 3.9.1 may be a bug fix release but I see fixing server info handling as one of the "major selling points" of the release!

[hoffie]

Rebased and un-draft-ed as #2829 has been merged.
(I'm planning to work on #2845 in a seperate PR)

[ann0see]

Besides my comment, happy to approve.

[hoffie]

Opened jamulussoftware/jamuluswebsite#822 for the documentation. Therefore removing the needs-documentation label here. As stated, this is totally optional and doesn't have to be integrated at 3.9.1 release time.

[ann0see]

As it's small, I've merged it.