feat: revert keyrings

[mattsb42-aws]

Description of changes:

Because of the delays that we introduced that are blocking the release of keyrings[1], this reverts the keyring changes to unblock releases between now and the keyring release. Keyring work is now present over on the keyring branch.

NOTE: This revert is messy in no small part because the keyring branch lived for so long without merging back into master. One of the affects of this is that we completely changed how a lot of our CI worked in that branch, and that carried over into master when we merged. This is why this "revert" PR retains the CI changes: both because the user experience of the new CI is much better and because our branch protection rules now require these 70+ checks and temporarily removing that requirement is non-trivial.

[1] https://github.com/awslabs/aws-encryption-sdk-specification/milestone/1

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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• Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

[seebees]

LGTM