csb_PL: Add alternative month names (bug 23140).

Kashubian language is not supported by CLDR, data copied from Wikipedia
and documents released by RJK (official Kashubian Language Council),
also consulted with a native speaker.

Note that this language also needs ab_alt_mon feature due to the month
May: nominative "môj", genitive "maja"; abbreviated nominative "môj",
abbreviated genitive "maj".

	[BZ #23140]
	* localedata/locales/csb_PL (mon): Rename to...
	(alt_mon): This.
	(abmon): Rename to...
	(ab_alt_mon): This.
	(mon): Add with proper genitive forms, copy from Wikipedia.
	(abmon): Likewise.