Rename PCF auth plugin to CF

[tyrannosaurus-becks]

This PR renames the PCF auth plugin to CF, Cloud Foundry's current preference. The plugin repository on which it depends has already undergone renaming.

In keeping with the example set by the new TokenUtil fields, this PR is fully backwards-compatible. For now, it supports both "pcf" and "cf" as commands. However, all documents are updated to the new method, and all PCF fields are marked for future deprecation, which we can do at an appropriate time.

This PR has been tested in the following manner:

• A full run of it from the CLI using the original manner of enabling it, configuring it, adding a role, and logging in.
• A full run of it from the CLI using the new manner of the same.
• A full run of it from the CLI using a hybrid manner where it was enabled in the past way but used in the modern way.

[kalafut]

This looks good. A couple of other related items outside the scope of the PR would be:

• update old changelog references
• request to update the blog post (maybe?)

[malnick]

Thank you!