feat!: deprecate CordovaPlugin's method initialize

[erisu]

Motivation, Context & Description

It is recommended to use pluginInitialize and not initialize.

At one point initialize was deprecated but later the flag was removed. This PR is re-adding the flag to make aware that the initialize method is deprecated and will be removed in a future release. Plugin developers should switch to pluginInitialize.

The question about this originated from doc repo: apache/cordova-docs#1386

Testing

npm t

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.50%. Comparing base (dff2fc6) to head (9a2ef2b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1771   +/-   ##
=======================================
  Coverage   72.50%   72.50%           
=======================================
  Files          23       23           
  Lines        1837     1837           
=======================================
  Hits         1332     1332           
  Misses        505      505           

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