renamed '=' to '=copy' [backport:1.2] (#15585)

* Assign hook name changed to `=copy`
* Adapt destructors.rst
* [nobackport] Duplicate tests for =copy hook
* Fix tests
* added a changelog entry

Co-authored-by: Clyybber <darkmine956@gmail.com>

changelog.md

@@ -215,6 +215,10 @@
 
 - The `=destroy` hook no longer has to reset its target, as the compiler now automatically inserts
   `wasMoved` calls where needed.
+- The `=` hook is now called `=copy` for clarity. The old name `=` is still available so there
+  is no need to update your code. This change was backported to 1.2 too so you can use the
+  more readability `=copy` without loss of compatibility.
+
 - In the newruntime it is now allowed to assign to the discriminator field
   without restrictions as long as case object doesn't have custom destructor.
   The discriminator value doesn't have to be a constant either. If you have a