Reverts flutter/flutter#140478
Initiated by: loic-sharma
This change reverts the following previous change:
Original Description:
1. Change templates to `IPHONEOS_DEPLOYMENT_TARGET`, `MinimumOSVersion`, and Podfile `platform :ios` to 12.0.
2. Add migrator for Podfile part to migrate `platform :ios, '11.0'` -> `platform :ios, '12.0'`
3. Compile with `-miphoneos-version-min=12.0`
4. Run the migrator on all example apps and integration tests.

See also https://github.com/flutter/flutter/pull/62902 and https://github.com/flutter/flutter/pull/85174 and https://github.com/flutter/flutter/pull/101963

Fixes https://github.com/flutter/flutter/issues/136060