api: Deduplicate alerts definition via allOf parameter

[mxinden]

Now that the fix for OpenAPITools/openapi-generator#1465 was merged, we can deduplicate the alert definition in the openapi.yaml spec like we did for silences.

More details:

alertmanager/api/v2/openapi.yaml

Lines 355 to 359 in 091a8a8

	# Shared properties in gettableAlert and postableAlert could also be
	# de-duplicated via the `allOf` parameter like it is done with silences. So
	# far this is blocked as the elm generator does not support it. For more
	# details see https://github.com/OpenAPITools/openapi-generator/issues/1465.
	gettableAlert: