Refs #19080

Refs #18767
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Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.

Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x

Introduce new variable: $breadcrumb-item-padding

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Fixes #18505
Closes #18509

Refs #18110

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Refs #18459, #17178
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See a8dc4812#commitcomment-14677726

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[V4 Docs] Update migration docs to point out the .btn-default replacement.

V4 Docs - Updates to migration docs to pointed out the removal of .btn-default class and an addition of the new .btn-secondary class.

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Calls out the new `.table-inverse`, `.table-reflow`, `.thead-default` and `.thead-inverse`

Audit the new table variants in V4 that aren't in V3

X-Ref: #17120 for `.panel-default` replacement

Example:
> ... SCSS variables (e.g., `@enable-gradients: true`) ...
Referenced SCSS but used the LESS syntax. Changed to:
> ... SCSS variables (e.g., `$enable-gradients: true`) ...

Closes #17114