[BUG] [CSharp] Use lowercase string.Join() instead of String.Join()

Bug Report Checklist

• [v] Have you provided a full/minimal spec to reproduce the issue?
• [v] Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?

Not with master, but with the latest released version in npm.

• [v] Have you searched for related issues/PRs?
• [v] What's the actual output vs expected output?

Description

In CSharp code generation, there is a build error in generated code due to String.Join function being used with the UpperCase String which is conflicting with a custom String class defined in the generated code (bad practice, but it is out of my control, I didn't define the json schema, just generating code for it), this would not happen if string lowercase was used, since the class names start with UpperCase.

openapi-generator version

openapi-generator-cli version
Did set selected version to 7.8.0

Steps to reproduce

openapi-generator-cli generate -g csharp --skip-validate-spec  -i https://raw.githubusercontent.com/PagerDuty/api-schema/5d679f79622d30b767c777be5f2e9787ef21e092/reference/REST/openapiv3.json

In the code generated, if you search the project by "String.Join" with case sensitive search, it will show the invalid code references.

Suggest a fix

When generating code, use always
string.Join( with the lowercase s instead of String.Join( with the uppercase S.

Start by running the provided openapi-generator-cli command with generator version 7.8.0, then search the generated C# project for case-sensitive "String.Join" references. Trace those references back to the generator source or templates. Done means generated C# uses lowercase "string.Join(" and no conflicting uppercase references remain.