Move values the mainmenu caches to dedicated file(s)

[sfan5]

It always bothered me a bit that Minetest was keeping information like the last update check in the user configuration file, but now I have a better solution to offer.
This PR moves such values to a dedicated conf file in the user's cache path.

Benefits:

• Server list geoip and CDB update json can be cached
• Clearing the cache makes MT recheck such things, which is reasonable behaviour
• The settings disappear from minetest.conf.example and the settings tab. They weren't meant to be user-configurable anyway
• Less load on CDB due to Minetest clients checking for package updates

Technical:

$path_cache/common.conf e.g.:

geoip_last_checked = 1709577888
cdb_updates_last_checked = 1709577712
no_mtg_notification = true
update_last_checked = 1709577794
geoip = EU

$path_cache/cdb/updates.json: contains the API response

To do

This PR is Ready for Review.

How to test

1. launch Minetest with --verbose to see HTTP requests made
2. look at files in cache path

[SmallJoker]

geoip cache works
cdb cache works
no_mtg_notification is set correctly
code looks good
👍