Update default variant to aws-k8s-1.21

[tjkirch]

Description of changes:

• Update Makefile.toml to build aws-k8s-1.21 by default instead of 1.19
• Update documentation and examples to reference aws-k8s-1.21
• Make sources/models/src/aws-k8s-1.21/mod.rs the canonical k8s model by making 1.19 link to 1.21 instead of vice versa, without changing the file contents (we symlink aws-k8s-*/mod.rs to a single file, and reference that one file from sources/models/README.md, so it's readable in GitHub, since symlinks aren't)

For reference, the last update to a 1.19 default was here: #1394

Testing done:

Confirmed cargo make builds a working 1.21. Also confirmed 1.19 builds are still healthy.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[tjkirch]

^ This push rebases on develop to fix a conflict with #1629. I retested 1.19 and 1.21. git was a little angry about it because of the file type change to sources/models/src/aws-k8s-1.{19,21}/mod.rs; @etungsten would you mind double-checking my work?