PSA: the generated `clone()` and `copyWith()` methods are deprecated

As a general PSA, the generated `MyMessage.clone()` and `MyMessage.copyWith(Function updates)` methods are deprecated. These methods can add significant overhead to your binary; we plan to remove in a future major version release of this package.

Instead of `MyMessage.clone()`, call `GeneratedMessageGenericExtensions.deepCopy()`.

Instead of `MyMessage.copyWith()`, call `GeneratedMessageGenericExtensions.rebuild()`.

We'll link to this issue from the generated source.

Start by locating the generated source referenced in the issue and check how deprecations for MyMessage.clone() and MyMessage.copyWith(Function updates) are presented. Confirm whether the issue URL is linked there and whether the documented replacements, deepCopy() and rebuild(), are clear; done means the generated-source reference communicates the migration path.

Written by the indexing model from the issue text.