Use simplified babel plugin on ember 3.27+ and drop unsupported node versions

[ef4]

This accompanies ember-cli/babel-plugin-htmlbars-inline-precompile#462.

[ef4]

This is now ready to go:

• it uses the new simplified babel-plugin-ember-template-compilation on ember 3.27+
• it still supports the same range of older ember versions as before, using babel-plugin-htmlbars-inline-precompile
• we dropped node < 12
• I upgraded the test setup dependencies enough to get tests passing under latest canary
• since ember 4 drops "legacy " AST plugin support, I removed those tests.
• this includes a full regeneration of the yarn.lock, so merging it should close the backlog of dependabot PRs.
• it seems that the compiled output of decorators was hard-coded into two tests and changed, so I had to change them to follow. As far as I can tell this is benign.

[NullVoxPopuli]

what's the simplified babel plugin do? (tldr?)

[ef4]

It implements inline template compilation and only that. And it does it in a way that composes with other Babel plugins that want to emit inline templates.

[mattmcmanus]

@NullVoxPopuli You can read the write up in the linked PR for a more detailed overview