chore(scss): sync Bootstrap variables with 5.3.4 deprecations

Aligns _bootstrap-variables.scss with the additions and deprecations
introduced in Bootstrap 5.3.4:

Added missing null variables:
- \$btn-close-filter
- \$carousel-control-icon-filter

Marked deprecated-in-5.3.4 variables with inline comments so users
migrating from older Bootstrap minors can see they're deprecated:
- \$btn-close-white-filter
- \$carousel-dark-indicator-active-bg
- \$carousel-dark-caption-color
- \$carousel-dark-control-icon-filter

Documented the intentional override of \$modal-content-color (null in
AdminLTE vs var(--bs-body-color) in Bootstrap) so the deviation is
self-explanatory.

src/scss/_bootstrap-variables.scss

@@ -1517,6 +1517,8 @@ $modal-dialog-margin-y-sm-up:       1.75rem !default;
 
 $modal-title-line-height:           $line-height-base !default;
 
+// AdminLTE keeps `$modal-content-color: null` so the modal text colour is
+// inherited from the document. Bootstrap's default is `var(--#{$prefix}body-color)`.
 $modal-content-color:               null !default;
 $modal-content-bg:                  var(--#{$prefix}body-bg) !default;
 $modal-content-border-color:        var(--#{$prefix}border-color-translucent) !default;
@@ -1684,12 +1686,16 @@ $carousel-control-next-icon-bg:      url("data:image/svg+xml,<svg xmlns='http://
 
 $carousel-transition-duration:       .6s !default;
 $carousel-transition:                transform $carousel-transition-duration ease-in-out !default; // Define transform transition first if using multiple transitions (e.g., `transform 2s ease, opacity .5s ease-out`)
+$carousel-control-icon-filter:       null !default;
 // scss-docs-end carousel-variables
 
 // scss-docs-start carousel-dark-variables
-$carousel-dark-indicator-active-bg:  $black !default;
-$carousel-dark-caption-color:        $black !default;
-$carousel-dark-control-icon-filter:  invert(1) grayscale(100) !default;
+// Carousel "dark" variables below have been deprecated in Bootstrap 5.3.4.
+// Use the `.carousel` colour-mode rules and `$carousel-control-icon-filter`
+// instead.
+$carousel-dark-indicator-active-bg:  $black !default; // Deprecated in 5.3.4
+$carousel-dark-caption-color:        $black !default; // Deprecated in 5.3.4
+$carousel-dark-control-icon-filter:  invert(1) grayscale(100) !default; // Deprecated in 5.3.4
 // scss-docs-end carousel-dark-variables
 
 
@@ -1722,7 +1728,8 @@ $btn-close-opacity:          .5 !default;
 $btn-close-hover-opacity:    .75 !default;
 $btn-close-focus-opacity:    1 !default;
 $btn-close-disabled-opacity: .25 !default;
-$btn-close-white-filter:     invert(1) grayscale(100%) brightness(200%) !default;
+$btn-close-filter:           null !default;
+$btn-close-white-filter:     invert(1) grayscale(100%) brightness(200%) !default; // Deprecated in 5.3.4. Use `$btn-close-filter` with the `.btn-close` colour-mode rules instead.
 // scss-docs-end close-variables