Fixes for Java codegen

[mr-c]

• java codegen: use double quotes for string literals in generated code.
• java codegen: UnionLoader.alternates needs a concrete collection type.

[mr-c]

Examples using CWL v1.2 & v1.2 schema is at common-workflow-language/cwljava#192

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.30%. Comparing base (6c06ff4) to head (c6d16be).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #912   +/-   ##
=======================================
  Coverage   83.30%   83.30%           
=======================================
  Files          22       22           
  Lines        4949     4949           
  Branches     1185     1185           
=======================================
  Hits         4123     4123           
  Misses        548      548           
  Partials      278      278           

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