[Java][RestTemplate] fix mustache template doesn't use reserved words for local variables

[amra]

Renaming local variable names.

fixes #11579: Java RestTemplate Mustache template doesn't use reserved words for local variables

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 (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[amra]

@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 please take a look on this PR

I started this on the master branch. I am going to rebase it on to 5.4.x.

[wing328]

Tested locally and the result is ok:

[INFO] --- maven-source-plugin:2.2.1:jar-no-fork (attach-sources) @ petstore-resttemplate-withxml ---
[INFO] Building jar: /Users/williamcheng/Code/openapi-generator/samples/client/petstore/java/resttemplate-withXml/target/petstore-resttemplate-withxml-1.0.0-sources.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  47.291 s
[INFO] Finished at: 2022-02-14T10:58:34+08:00
[INFO] ------------------------------------------------------------------------

[wing328]

@amra thanks for the PR, which has been merged into master.