Nation translation_domain "freeciv-core"
When translation domain "freeciv" was renamed as "freeciv-core", nation rulesets, or related code in ruleset.c, was not updated.
By luck, it's not really broken, and thus I'm not targeting fix to S3_0 (that is already d3f). Though the nation ruleset files set the nonexistent domain "freeciv", that's also what ruleset.c checks for the default to be, and then does the right thing.
For later branches we should change the domain name in nation ruleset files to match the actual domain name, i.e., "freeciv-core".
As part of #43200, this has also been posted to http://forum.freeciv.org/f/viewtopic.php?f=14&t=92040
New patch versions handle also those rulesets that have their own nations included, instead of using default nations from data/nations/
civ1 ruleset does not load with this.
New version fixes civ1 ruleset
At the same time this goes in, master needs another commit updating S3_1 commit hash to make the ruleup tests from. That commit cannot be prepared in advance, as the hash will be known only when S3_1 change has been pushed.