CREDITS: Add -next to Stephen Rothwell's entry

Stephen retired and stepped back from -next maintainership, update his
entry in CREDITS to recognise his 18 years of hard work making it what
it is today and all the impact it's had on our development process.

Also update to his current GnuPG key while we're here.

Acked-by: Stephen Rothwell <[e-mail]>
Acked-by: SeongJae Park <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

CREDITS

···
 N: Stephen Rothwell
 E: [e-mail]
 W: http://www.canb.auug.org.au/~sfr
-P: 1024/BD8C7805 CD A4 9D 01 10 6E 7E 3B  91 88 FA D9 C8 40 AA 02
+P: 4096R/5AD24211C060D1C8 D41C A3ED 5B30 275C F5A0  1B05 5AD2 4211 C060 D1C8
+D: Created linux-next and maintained it 2008-2026
 D: Boot/setup/build work for setup > 2K
 D: Author, APM driver
 D: Directory notification