bump pickleformat minor version

[adriaanm]

e.g. dotty would like to distinguish 2.11 from 2.12 to interoperate with the trait encoding

[DarkDimius]

Yes, please :-)

[SethTisue]

@adriaanm still relevant? something to get in to 2.13.0-RC1?

[dwijnand]

Better safe than sorry (adding it to the RC1 milestone)

[smarter]

the pickle format is 5.2 in 2.13 and 5.0 in 2.12, so it's already different (and it doesn't really matter for dotty, because we only target one version of scalac at a time)