[BSv5] replace lighten() by tint-color(), and var/func clean up

[chalin]

• Contributes to Upgrade to Bootstrap 5.2 #470
• Replaces custom coloring of Draw.io edit button by outline-primary:
  • Before:
  • After: see below
  • Reference: https://www.docsy.dev/docs/adding-content/diagrams-and-formulae/#diagrams-with-diagramsnet
• Drops (now) unused, elements (breaking changes). I've scanned all of the Docsy-based projects that I work on, and none use these elements:
  • $primary-light
  • color-diff()

Draw.io edit-button screenshot

[LisaFC]

I think the button change is fine, I can't imagine anyone's look and feel being absolutely wedded to having a red edit button, and the new version will match the surrounding site.

Have we checked any of our contributed examples for those elements?

[chalin]

Have we checked any of our contributed examples for those elements?

I've sampled the projects listed under Examples and none use these two SCSS elements. Of course, any project that might happen to use these elements can define their own after this PR is merged -- e.g., by copying the definitions that Docsy gave. The point that we're making here in dropping these elements is to recognize that they aren't a necessary part of Docsy (they weren't even mentioned in the User Guide).

[chalin]

Oops, I had forgotten to commit the removal of color-diff(). Done now.

[chalin]

Possible followup:

• Drop / inline prepend() SCSS function #1385