Fix warning and note styling that GitHub broke

GitHub changed their custom Markdown syntax for alert blocks.

README.md

@@ -71,7 +71,7 @@ Sanitize can sanitize the following types of input:
 * Standalone CSS stylesheets
 * Standalone CSS properties
 
-> **Warning**
+> [!WARNING]
 >
 > Sanitize cannot fully sanitize the contents of `<math>` or `<svg>` elements. MathML and SVG elements are [foreign elements](https://html.spec.whatwg.org/multipage/syntax.html#foreign-elements) that don't follow normal HTML parsing rules.
 >
@@ -419,13 +419,13 @@ elements not in this array will be removed.
 ]
 ```
 
-> **Warning**
+> [!WARNING]
 >
 > Sanitize cannot fully sanitize the contents of `<math>` or `<svg>` elements. MathML and SVG elements are [foreign elements](https://html.spec.whatwg.org/multipage/syntax.html#foreign-elements) that don't follow normal HTML parsing rules.
 >
 > By default, Sanitize will remove all MathML and SVG elements. If you add MathML or SVG elements to a custom element allowlist, you must assume that any content inside them will be allowed, even if that content would otherwise be removed or escaped by Sanitize. This may create a security vulnerability in your application.
 
-> **Note**
+> [!NOTE]
 >
 > Sanitize always removes `<noscript>` elements and their contents, even if `noscript` is in the allowlist.
 >