[SpringCloud] Fix parameter names with {{baseName}}

[ePaul]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change.
  → bin/spring-cloud-feign-petstore.sh
• Filed the PR against the correct branch: master for non-breaking changes (???)

Description of the PR

(Note that this includes the changes from #4901 for updating the samples before doing any changes. After merging those (and potentially rebasing this) the diff will look better in the samples part. I'm opening the PR already now to let CI have a look at it.)

This is a part of a series of fixes for #4898.
I think it fixes the wrong name translations for non-file-upload form parameters in spring-cloud library variants of spring language, when the original parameter name doesn't fit the naming conventions (e.g. camelCase) or conflicts with a reserved word. For file upload parameters, previously only the name file was accepted (now every name), but then translated to the baseName (instead of paramName).

The petstore example doesn't seem to have any occurrence where this actually makes any difference in the generated code. Or it could be that formParams.mustache is not used at all?

This needs Review + Tests by Java/Spring-Cloud/Feign experts. /cc @cbornet

[ePaul]

Rebased onto master after #4901 was merged.

[wing328]

@ePaul thanks for the fix, which looks good to me.