[Post-6.0] Fix up .spi.yml

[grynspan]

Per @finestructure 's comment here:

[...] (And drop the swift_version: 6.0 line once Swift 6.0 is released.)

[stmontgomery]

Tracked internally as rdar://129677005

[grynspan]

@finestructure Should we do this yet? :)

[finestructure]

Feel free, we've been building docs with Swift 6 by default for quite a while now!

[finestructure]

#908 :)

[stmontgomery]

Resolved by #908