Deprecate Dial and DialContext through a mechanism that doesn't trigger linters about grpc-go

[dfawley]

there are 178,000 usages

The go stdlib ioutil package, deprecated in 2022, is still imported by almost 700k packages: https://pkg.go.dev/io/ioutil?tab=importedby

We're following all the guidelines here as best we can. I'm sorry this has inconvenienced you, but you probably do need a way to allow the use of deprecated features in your presubmits, so you don't have to immediately update >100 call sites at the same time you update a dependency. For example, we have a list of exceptions here:

grpc-go/scripts/vet.sh

Lines 123 to 125 in 0020ccf

	# Only ignore the following deprecated types/fields/functions and exclude
	# generated code.
	noret_grep "(SA1019)" "${SC_OUT}" | not grep -Fv 'XXXXX PleaseIgnoreUnused

An explanation of this change and why we made it can be found in https://github.com/grpc/grpc-go/blob/master/Documentation/anti-patterns.md. Let us know if you have any questions or other concerns about it.