[Breaking Change] Text editor caret height will be based on strut height

[Jason Simmons]

Summary

The height of the caret in text field widgets will be changed to match the height of the strut.  This will result in a caret whose height matches the full height of the line of text.

Previously caret height was based on the height of the character next to the caret.

See https://github.com/flutter/flutter/pull/50354

Justification

This change will ensure that the caret height is consistent throughout the line and does not exceed the bounds of the line.  Carets outside line bounds can result in rendering errors such as missing selection handles (https://github.com/flutter/flutter/issues/40633).

The difference will not be noticeable in most applications.  Golden image tests that draw carets may need to be updated.