Update some WCAG links, expand sass.md contrast explanation

Author: patrickhlauke

scss/_functions.scss

@@ -177,9 +177,9 @@ $_luminance-list: .0008 .001 .0011 .0013 .0015 .0017 .002 .0022 .0025 .0027 .003
   @return if($l1 > $l2, divide($l1 + .05, $l2 + .05), divide($l2 + .05, $l1 + .05));
 }
 
-// Return WCAG2.0 relative luminance
-// See https://www.w3.org/WAI/GL/wiki/Relative_luminance
-// See https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests
+// Return WCAG2.1 relative luminance
+// See https://www.w3.org/TR/WCAG/#dfn-relative-luminance
+// See https://www.w3.org/TR/WCAG/#dfn-contrast-ratio
 @function luminance($color) {
   $rgb: (
     "r": red($color),

site/content/docs/5.2/components/dropdowns.md

@@ -14,7 +14,7 @@ Dropdowns are built on a third party library, [Popper](https://popper.js.org/),
 
 ## Accessibility
 
-The [<abbr title="Web Accessibility Initiative">WAI</abbr> <abbr title="Accessible Rich Internet Applications">ARIA</abbr>](https://www.w3.org/TR/wai-aria/) standard defines an actual [`role="menu"` widget](https://www.w3.org/WAI/PF/aria/roles#menu), but this is specific to application-like menus which trigger actions or functions. <abbr title="Accessible Rich Internet Applications">ARIA</abbr> menus can only contain menu items, checkbox menu items, radio button menu items, radio button groups, and sub-menus.
+The [<abbr title="Web Accessibility Initiative">WAI</abbr> <abbr title="Accessible Rich Internet Applications">ARIA</abbr>](https://www.w3.org/TR/wai-aria/) standard defines an actual [`role="menu"` widget](https://www.w3.org/TR/wai-aria/#menu), but this is specific to application-like menus which trigger actions or functions. <abbr title="Accessible Rich Internet Applications">ARIA</abbr> menus can only contain menu items, checkbox menu items, radio button menu items, radio button groups, and sub-menus.
 
 Bootstrap's dropdowns, on the other hand, are designed to be generic and applicable to a variety of situations and markup structures. For instance, it is possible to create dropdowns that contain additional inputs and form controls, such as search fields or login forms. For this reason, Bootstrap does not expect (nor automatically add) any of the `role` and `aria-` attributes required for true <abbr title="Accessible Rich Internet Applications">ARIA</abbr> menus. Authors will have to include these more specific attributes themselves.
 

site/content/docs/5.2/customize/sass.md

@@ -215,9 +215,9 @@ In practice, you'd call the function and pass in the color and weight parameters
 
 ### Color contrast
 
-In order to meet [WCAG 2.0 accessibility standards for color contrast](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html), authors **must** provide [a contrast ratio of at least 4.5:1](https://www.w3.org/WAI/WCAG20/quickref/20160105/Overview.php#visual-audio-contrast-contrast), with very few exceptions.
+In order to meet the [Web Content Accessibility Guidelines (WCAG)](https://www.w3.org/TR/WCAG/) contrast requirements, authors **must** provide a minimum [text color contrast of 4.5:1](https://www.w3.org/TR/WCAG/#contrast-minimum) and a minimum [non-text color contrast of 3:1](https://www.w3.org/TR/WCAG/#non-text-contrast), with very few exceptions.
 
-An additional function we include in Bootstrap is the color contrast function, `color-contrast`. It utilizes the [WCAG 2.0 algorithm](https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests) for calculating contrast thresholds based on [relative luminance](https://www.w3.org/WAI/GL/wiki/Relative_luminance) in a `sRGB` color space to automatically return a light (`#fff`), dark (`#212529`) or black (`#000`) contrast color based on the specified base color. This function is especially useful for mixins or loops where you're generating multiple classes.
+To help with this, we included the `color-contrast` function in Bootstrap. It uses the [WCAG contrast ratio algorithm](https://www.w3.org/TR/WCAG/#dfn-contrast-ratio) for calculating contrast thresholds based on [relative luminance](https://www.w3.org/TR/WCAG/#dfn-relative-luminance) in an `sRGB` color space to automatically return a light (`#fff`), dark (`#212529`) or black (`#000`) contrast color based on the specified base color. This function is especially useful for mixins or loops where you're generating multiple classes.
 
 For example, to generate color swatches from our `$theme-colors` map: