fix: [typescript-angular] `.join` does not exist on type `Set`

Created by: snebjorn

Fixes #11782 (closed) see bug report for details about the issue

To validate the work run

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
   -i https://raw.githubusercontent.com/snebjorn/openapi-gen-bug/master/join-bug.json \
   -g typescript-angular \
   -o output-master/join-bug

and check that all .join calls in a service (api/default.service.ts) are valid typescript. Meaning that it no longer calls .join directly on a Set.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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