Add headers (h1-h6) tag support for the new timeline renderer (#7754)

* Add headers (`h1-h6`) tag support for the new timeline renderer

* Make H1 and H2 use the same (smaller) style

* Update screenshots

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Co-authored-by: ElementBot <android@element.io>

libraries/matrixui/src/main/kotlin/io/element/android/libraries/matrix/ui/messages/ToHtmlDocument.kt

@@ -89,6 +89,12 @@ private object CustomHtmlToDomParser {
             "blockquote",
             "p",
             "br",
+            "h1",
+            "h2",
+            "h3",
+            "h4",
+            "h5",
+            "h6",
             // Add custom `mx-reply` tag, even if it's just to remove its contents from the plain text version of the message
             "mx-reply"
         )