Navbar fixes

This PR builds on @vanduynslagerp's work in #21719 as a basis for many of our navbar related issues. 😅 Basically there's a bug in a Sass mixin, some unnecessary position: absolute, a missing .container-fluid, and some inconsistent sizing of navbar sub-components. That's all getting fixed here.

• Correct navbar collapsing breakpoints resulting from the broken Sass mixin that was generating our .navbar-toggleable classes. Basically this wasn't generating mobile-first classes, so we were acting on the wrong media query values. Fixes #21691 (closed), fixes #21706 (closed), and closes #21719.

  As part of this, .navbar-toggleable classes have been renamed to .navbar-expand to match the mobile-first approach used elsewhere.

• Redo the .navbar-toggler alignment by dropping the modifiers entirely. Instead of using absolute position, we run with the DOM order and make use of justify-content: space-between, forcing navbar brand and toggler to be on opposite ends of the navbar. This drops .navbar-toggler-left and .navbar-toggler-right Fixes #21659 (closed), fixes #21735 (closed), fixes #21846 (closed), fixes #21984 (closed), and closes #21729.

• Resolve issues with .container-fluid not being fully supported within .navbar. Fixes #21530 (closed) and closes #21901.

• Normalizes the sizing of navbar brand, text, and link elements by pulling in the changes from #21721 (which fixes #21705 (closed), fixes #21956 (closed), and fixes #21604 (closed)). This one might still need further love, or our inputs will; the text input and buttons are 38px vs the nav's 40px. Any changes to that will come in another PR.

Still to do:

• Update navbar doc examples to drop toggler modifiers and reorder brand/toggler
• Drop navbar toggler variants in docs