Terrain & imagery provider ready deprecation fixes

[ggetz]

• Fixes "ready" is deprecated, but is broken #11274
  • This is resolved by ensuring the existence of either _ready or ready before checking the value.
  • The logic is a bit more complicated in these checks as we don't want to check ready unless completely necessary. Otherwise an unneeded deprecation warning is thrown when accessing ready for our build-in providers.
• Fixes deprecation warning typo reported in Remove readyPromise and related properties in 1.107 #11195 (comment)
• Fixes Cesium reference docs host old versions of the documentation in certain pages. #11303 (comment)

For testing, here's a local sandcastle with custom terrain and imagery providers. Sandcastles related to terrain or imagery should also be verified.

[cesium-concierge]

Thanks for the pull request @ggetz!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[jjhembd]

Thanks @ggetz, this looks good overall. I had one question

[jjhembd]

Looks good @ggetz! Merging now