wifi: cfg80211: rename UHB to 6 GHz

UHB stands for "Ultra High Band", but this term doesn't really
exist in the spec. Rename all occurrences to "6 GHz", but keep
a few defines for userspace API compatibility.

Link: https://msgid.link/20240206164849.c9cfb9400839.I153db3b951934a1d84409c17fbe1f1d1782543fa@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>