Improve version management, update dependencies, remove unused dependencies

[agilob]

Goals of this PR:

• general improvement of version management
• • centralise dependency management in main pom.xml for dependencies/plugins that are used in more than one module
• remove dependencies and plugins that exist in classpath in more than one version
• upgrade some dependencies to newer versions that are java 11+ ready

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.
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• File the PR against the correct branch: master (5.3.0), 6.0.x
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[agilob]

Im going to check for unused dependencies now using mvn dependency:analyze

and found these:

WARNING] Unused declared dependencies found:
[WARNING]    org.springframework.boot:spring-boot-starter-web:jar:2.2.9.RELEASE:compile
[WARNING]    com.fasterxml.jackson.datatype:jackson-datatype-jsr310:jar:2.10.2:compile
[WARNING]    com.fasterxml.jackson.core:jackson-core:jar:2.10.2:compile
[WARNING]    org.springframework.boot:spring-boot-starter-test:jar:2.2.9.RELEASE:test
[WARNING]    org.springframework.boot:spring-boot-test:jar:2.2.9.RELEASE:test
[WARNING]    org.hamcrest:java-hamcrest:jar:2.0.0.0:test

each will be removed in separate commit for easy reversal, but pushed together

[agilob]

org.springframework.boot:spring-boot-starter-web:jar:2.2.9.RELEASE:compile

mvn plugin reports this as unused, false positive :(

[agilob]

@wing328 @jimschubert I think that's it 👍

[wing328]

cc @OpenAPITools/generator-core-team