Changes the name of Perl 6 to Raku (#3312)

Since October 2019, Raku is the name of the language formerly known as
Perl 6. This reflects the change. It's the same language, so changes
are mostly cosmetic.

Perl6.gitignore → Raku.gitignore

-# Gitignore for Perl 6 (http://www.perl6.org)
+# Gitignore for Raku (https://raku.org)
 # As part of https://github.com/github/gitignore
 
 # precompiled files