megadriv.xml: fix release info node

[Angelo Salese]

Has inconsistencies of datetime format:

• a regular YYYYMMdd 19900309;
• a variant for world that appends above with a (JPN) suffix (pretty ugly as data is concerned);
• various dd-MM-YYYY 20-08-2020;
• various YYYY-MM-dd 2020-03-04;

[Deleted user]

Should be done for the mainlsit too: years with "?", "??" etc etc
No consistent naming for the years.

[Vas Crabb]

The YYYYMMdd format is the one we favour in software lists. The lists with the most rigorously verified information use that format already (mostly Japanese home computer software lists).