☂️ Increase minimum support iOS version from 9 to 11

[jmagman]

Increase minimum target iOS version from 9.0 to 11.0 flutter.dev/go/rfc-32-bit-ios-unsupported.
In particular, this will let us drop OpenGL support on iOS in favor of Metal.

• Increase app minimum supported iOS version from 9.0 to 11.0 #101960
• Enforce minimum plugin version iOS 11 #103545
• Framework->plugins roller failing on deprecation error #103912
• Increase plugin template minimum supported iOS version from 9.0 to 11.0, match Flutter SDK constraint #101961
• Set buildroot ios_deployment_target to 11.0 #101964
• Investigate Metal on iOS without OpenGL fallback. #52862
• Roll buildroot to target minimum iOS 11 engine#33401
• Remove availability and fallbacks for iOS 11 and macOS 10.13 engine#34919
• Update all plugins minimum from iOS 9 to iOS 11 #101965
• Update iOS minimum version documentation to 11.0 website#7037

Cleanup:

• Remove iOS OpenGL backend #103929

See issues/PRs from when it was bumped 8.0->9.0
#62902
#68729
#84198
#85174
#89695
flutter/buildroot#509
flutter/engine#28572
flutter/engine#28743
flutter/website#6057

Though increasing the minimum from 9 to 11 will implicitly drop 32-bit iOS support, additional related work is tracked separately at #97327

[jmagman]

/cc @zanderso @chinmaygarde

[jmagman]

cc @xster for 1p app impact