Prepare for certbot >= v3.0.0, migrate from snap core20 to core24

[ngandrass]

Currently, certbot-dns-netcup is not compatible with certbot version >= 3.0.0. Executing certbot results in the following error message:

root@hostname:/# certbot --version
The following plugins are using an outdated python version and must be updated to be compatible with Certbot 3.0. Please see https://community.letsencrypt.org/t/certbot-3-0-could-have-potential-third-party-snap-breakages/226940 for more information:
  * certbot-dns-netcup
certbot 3.0.1

This PR contains the changes proposed in #35 to make this plugin compatible with the current certbot version.

A build using snapcraft v8.4.4 with core24 succeeded. However, I never worked with snap before so please cross check my changes!

[coldfix]

Hi,
many thanks for your PR!
I don't have much experience with snap either. In fact, I found it a rather inconvenient to work with, especially for plugin purposes..
Your commit adds a few sections to the snapcraft.yml. May I inquire if that was necessary to get it to build or what's the reasoning behind it?

[coldfix]

Closes #35

[ngandrass]

Your commit adds a few sections to the snapcraft.yml. May I inquire if that was necessary to get it to build or what's the reasoning behind it?

I adjusted the snapcraft.xml to match the configs that are generated by certbot/tools/snap/generate_dnsplugin_snapcraft.sh and manually checked against the migration guides from snap core20 to core22 and core22 to core24.

[coldfix]

Your commit adds a few sections to the snapcraft.yml. May I inquire if that was necessary to get it to build or what's the reasoning behind it?

I adjusted the snapcraft.xml to match the configs that are generated by certbot/tools/snap/generate_dnsplugin_snapcraft.sh and manually checked against the migration guides from snap core20 to core22 and core22 to core24.

Okay. I think I'm going with this smaller change for now, because I don't yet have enough reason to believe the other stuff is necessary.

Thanks for initiating!