[Templates] Undefined readOnly variable instead of the right isReadOnly

Description

This issue regards at least client documentation automatic generation (plus for some languages also not just doc) with swagger-codegen because of a bug in many languages templates.
Precisely, it regards the use of the undefined template variable readOnly to check if the variable/parameter is read-only while the right variable to use in the templates is isReadOnly as defined by: https://github.com/swagger-api/swagger-codegen/commit/a6df340cdc80f71cc4bdb88aa7174013a5d41c5c

Swagger-codegen version

I am using version 3.0.30 but I don't see this fixed in the current master.

Swagger declaration file content or url

Command line used for generation

swagger-codegen generate -i <your yaml file with models that have readonly attributes> -l python -o client

Note: it does not apply only to python clients.

Steps to reproduce

Just generate for example a python client.
You will see that for the generated models documentation, models' attributes that are readonly are not annotated as readonly because of the problem mentioned above.

Related issues/PRs

I haven't found any but please feel free to point that out if I missed something.

Suggest a fix/enhancement

Replace readonly with isReadOnly whenever it occurs in mustache templates.

Search the Mustache templates for uses of the undefined readOnly variable, starting with the templates involved in Python model documentation generation. Verify the generated documentation for a model with readonly attributes, then check other language templates to confirm the incorrect variable uses have been addressed.