feat(design)!: move DaffBackdropModule into DaffSidebarModule

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

DaffBackdropModule is its own component. It would not be used anywhere else, so it should be part of sidebar. I believe it was originally moved out of sidebar because we thought we'd use it in modal, but we've built modal to use cdkOverlay` instead.

Fixes: #2566

What is the new behavior?

Backdrop logic is now part of the DaffSidebarModule. DaffBackdropModule is removed from the design library.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information