Very similar to #148586 for newly created plugins (not apps or plugin example apps).

We have metrics that show in 2024 Q1 less than 8% of flutter create -t plugin plugins were Objective-C. Deprecate and remove the flag and templates once that number reaches < 3% (making that number up..)

• Add a deprecation warning that the flag will be removed in a future release. Let that sit for a stable release or so to see if we get complaints. Add deprecation warning for "flutter create --ios-language" #155867
• Remove the --ios-language flag and plugin templates from the tool.

Users will still be able to add Objective-C code to their plugin or replace the Swift code that's there with Objective-C.