Extend deprecation period for python_awslambda alias (#20101)

This PR extends the deprecation of the old `python_awslambda` alias for
the `python_aws_lambda_function` target, to make it easier/smoother for
people to upgrade to 2.18 and particularly 2.19.

We renamed the `python_awslambda` target to `python_aws_lambda_function`
in #19216 (in 2.18), and deprecated the old name. This was originally
scheduled to be removed in 2.19, and happened in #19759, but I've
reconsidered that slightly: 2.17 - 2.19 is a time of much upheaval for
Pants' FaaS support, so it seems sensible to try to smooth that over if
we can, especially for something like this, that's just two lines of
configuration (not even "real" code).

src/python/pants/backend/awslambda/python/target_types.py

@@ -177,6 +177,9 @@ def validate(self) -> None:
 class PythonAWSLambda(_AWSLambdaBaseTarget):
     alias = "python_aws_lambda_function"
 
+    deprecated_alias = "python_awslambda"
+    deprecated_alias_removal_version = "2.21.0.dev0"
+
     core_fields = (
         *_AWSLambdaBaseTarget.core_fields,
         PythonFaaSDependencies,