Bump pub_semver dependency

[creativecreatorormaybenot]

I think the pub get error message explains this issue best :)

build_runner >=1.12.0 depends on pubspec_parse ^1.0.0 which depends on pub_semver ^2.0.0, build_runner >=1.12.0 requires pub_semver ^2.0.0.
And because platform_detect 1.4.0 depends on pub_semver ^1.4.2 and no versions of platform_detect match >1.4.0 <2.0.0, build_runner >=1.12.0 is incompatible with platform_detect ^1.4.0.

Essentially, pub_semver should be bumped to 2.0.0.