exp/textinput: remove `Field` APIs added since v2.9.0

[hajimehoshi]

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

What feature would you like to be added?

Now we have Field.Generation, remove Field.ChangedAt. This was introduced at v2.9, so it's ok to remove it.

EDIT: Remove all the new methods of Field added since v2.9. Composer is the new API replacing Field.

Why is this needed?

No response

[hajimehoshi]

Also remove these deprecated methods:

• Field.WriteText
• Field.WriteTextRange
• Field.WriteTextForRendering
• Field.WriteTextForRenderingRange