MDL-80519 theme_boost: Add backward compatibility for BS4 SCSS

This commit will add a bs4-compat SCSS file with some of the helper classes
that are dropped in BS5. That will help third-party plugins to do the migration.

- Add backward compatibility for .media
- Add backward compatibility for badges
- Add backward compatibility for forms
- Add backward compatibility for cards
- Add backward compatibility for .no-gutters
- Add backward compatibility for spacing
- Add backward compatibility for margins and paddings
- Add backward compatibility for float directions
- Add backward compatibility for border directions
- Add backward compatibility for text directions
- Add backward compatibility for .close
- Add backward compatibility for font utilities
- Add backward compatibility for rounded sizes
- Add backward compatibility for sr-only
- Add backward compatibility for custom controls
- Add backward compatibility for .btn-block

.upgradenotes/MDL-80519-2025020702344655.yml

@@ -0,0 +1,7 @@
+issueNumber: MDL-80519
+notes:
+  theme_boost:
+    - message: >-
+        Added new bs4-compat SCSS file (initially deprecated) to help
+        third-party plugins the migration process from BS4 to BS5
+      type: deprecated

theme/boost/scss/moodle/deprecated.scss

@@ -46,3 +46,6 @@
     @include deprecated-styles();
     display: none;
 }
+
+// TODO: MDL-84465 Final deprecation in 6.0. Remove this line when the Bootstrap 4 backward compatibility period ends.
+@import "bs4-compat";