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.