Sunset cordova-plugin-file-transfer

Per CB-12711, we have decided to sunset the file transfer plugin in favour of new XHR (XHR2) features that should be able to provide everything needed to replace this plugin's functionality.

Initial steps:

1. Do a test of XHR2 features across main platforms to get a sense of what an API transition to XHR2 would look like. This could be a basis for a blog post in the future.
2. Write up a blog post on how to transition to XHR2, and what platform versions/requirements are necessary to make it happen.

Should the above look good, and a clear path forward exists, further todos (file as subtasks in here eventually):

3. Deprecate JIRA components.
4. Close out all github PRs and JIRA issues.
5. What do we need to inform Apache Infra of?
6. Update plugin README to reflect status of plugin and point to transition blog post.
7. Update coho to remove any references to the plugin.
8. Update cordova-docs to remove plugin references / remove code generation around cordova-plugin-file-transfer.
9. Do one final release of the plugin.
10. Ensure we deprecate the plugin in npm.