Drop support for Fedora 38

[almet]

Fedora 38 is EOL since 21 May 2024, so this removes the specific branches we had checking for it, and updates the related instructions.

[apyrgio]

I see two places that still refer to Fedora 38:

$ git grep -i fedora | grep 38
BUILD.md:   > :bulb: Alternatively, you can use your base Fedora 38 template in the
[...]
dev_scripts/qa.py:class QAFedora38(QAFedora):

The first should change to refer to Fedora 40. What we're saying to the user there is that they can install Dangerzone on their base Fedora 40 template, instead of a separate one. For the second reference, we should just remove it.

[almet]

🔖 Hey, thanks for catching this up. Mentally bookmarking the -i flag and the grep on the version number for later reference.

[apyrgio]

Oh nice, you found another instance of Fedora 38 as well. Thanks for the changes, feel free to merge.