mirrors.bbclass: update CPAN_MIRROR

Both of these redirect to https://cpan.metacpan.org/:
http://cpan.metacpan.org/
http://search.cpan.org/CPAN/



Signed-off-by: Tim Orling <tim.orling@konsulko.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>