Mark Slim3 server generator as deprecated

[ybelenko]

PR checklist

• Read the contribution guidelines.
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Abandon Slim3 generator in favor of Slim4. #3658
It doesn't make sense to enhance/support generator based on outdated framework.

cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @renepardon

[wing328]

Thanks for the PR.

Usually, we rename the generator with the suffix -deprecated as well, e.g. swift3-deprecated

[wing328]

@ybelenko thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569

[wing328]

Yup, just updated my reply.