Move to a translucent input border with background-clip: padding-box for improved blending across different background-colors

- Moves from no set line-height (inherited of 1.5) to declared 1.25 for
all inputs and buttons (regardless of size modifier).

- Updates padding to be `.5rem` instead of `.375rem` so that padding is
more likely to be whole numbers based on the root font-size.

- Whole numbers will be beneficial in avoiding odd fractional pixels
that can lead to misalignment as shown in #18607.

- Large buttons and inputs are now a tad wider, and smaller
buttons/inputs a tad shorter and narrower, too.

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- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

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Fixes #18609

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- Renames old and unused $nav-pills-active-link-hover-* for
$nav-pills-active-link-* ones
- Puts those vars to use in place of defaults $component-* vars on nav
pills

- Add new $nav-tabs-border-width for the bottom border on the .nav-tabs
parent class
- Use that new variable for the negative margin on nav-items within it
for consistent customization
- Drop the $nav-tabs-link-border-width for the new variable so it’s all
tied together