[templates] Update templates vendor ext standards

[jimschubert]

This updates the templates for a number of languages to follow the lower-kebab with x- prefix for vendor extensions as we've defined in our contributing documentation (and the x- prefix is required by OAI specification).

This fixes an issue I introduced in #5192 in which Haskell client is using additionalProperties as a sort of vendorExtension, and I'd worked to update all properties and found that those additionalProperties were not prefixed in templates with vendorExtensions.. It's possible that additionalProperties makes sense for these properties--I didn't dig into it.

The changes in this PR are simply moving to our convention. There should be no effect on template consumers/extenders as the old format vendor extensions still exist and there is a warning that the format has been deprecated in 4.3.0.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[jimschubert]

cc @OpenAPITools/generator-core-team

[jimschubert]

This PR is work toward #4976. Once merged, and master is integrated into the 5.x branch, we can remove the "old" format vendor extensions.