introduce oscal cli validation and remove legacy example projects

[wandmagic]

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[xee5ch]

Following up on https://github.com/GSA/fedramp-automation/pull/601#pullrequestreview-2129108508, I added https://github.com/oscal-club/oscal-cli-action/pull/10 to make how to run shell commands work, it is already supported. Let me know if you have other questions or interest in pivoting to the use of the oscal-cli action.

[david-waltermire]

@wandmagic Can you remove the web targets and schematron validation targets from the corresponding makefiles? We won't need these going forward.

[david-waltermire]

We should merge #604 first before this PR, which will fix most of the validation issues.