Generate setters for readonly properties in server code

Created by: tsiq-karold

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Fixes https://github.com/OpenAPITools/openapi-generator/issues/1376.

I explored a couple of other alternatives, namely passing the tag (i.e. CodegenType) to the template and switching there or adding another property to the data passed to the template to indicate that a setter should be generated. Unfortunately, they didn't really work or were horrendously messy.

Please note I have run only ./bin/jaxrs-petstore-server.sh which generated new methods which can be seen in this PR and ./bin/java-petstore-jersey2.sh which generated no changes as expected. This change does impact all generators and I ran ./bin/run-all-petstore locally successfully but the message that printed said not to run it. It also changed some 900 files under samples which I thought would massively pollute this PR. Happy to do that if it's necessary though.