update page transition to allow ^2.0.0

[AntonyLeons]

animated_splash_screen 1.1.0 depends on page_transition 2.0.1-nullsafety.0 and no versions of animated_splash_screen match >1.1.0 <2.0.0, animated_splash_screen ^1.1.0 requires page_transition 2.0.1-nullsafety.0.

null safety naming standards have changed, you no longer require the suffix.

page_transition: ^2.0.2 should be valid