Fix copy-pasted log message in SpanContext.__delattr__ (#5455)

* Fix copy-pasted log message in SpanContext.__delattr__

The debug message logged when deleting an attribute said "ignoring call
to set attribute", copied from __setattr__. Say "delete attribute" so
the log matches the intercepted operation.

* Add changelog fragment

* Rename changelog fragment to match new PR number

The original PR (#5439) was closed and recreated as #5455 because
its branch name contained a non-ASCII character.

* Apply ruff-format