Use the current names for the language config keys

Hugo 0.158.0 renamed two of the keys the German language is configured with and
warns about the old names on every build:

  WARN  deprecated: project config key languages.de.languageCode was deprecated
  WARN  deprecated: project config key languages.de.languageName was deprecated

Rename languageCode to locale and languageName to label. The generated site is
byte for byte the same afterwards, apart from the DTSTAMP of the calendar
feeds, which carries the time of the build.

Fixes: b62b53674f ("new theme (work in progress)")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

config/_default/languages.de.toml

@@ -1,6 +1,6 @@
 disabled = false
-languageCode = "de"
-languageName = "Deutsch"
+locale = "de"
+label = "Deutsch"
 weight = 1
 title = "Chaos Computer Club Berlin"