fix: remove softbreak from github legacy callout

- A softbreak was being preserved during the github legacy callout (this
is likely due to a change in Goldmark) while it should not. This caused
an `<br>` to be present between the attention title and attention content.
- Added unit test.

(cherry picked from commit 216a542bfb)

modules/markup/markdown/callout/github_legacy.go

@@ -63,6 +63,14 @@ func (g *GitHubLegacyCalloutTransformer) Transform(node *ast.Document, reader te
 			attentionParagraph.AppendChild(attentionParagraph, calloutNode)
 			firstParagraph.Parent().InsertBefore(firstParagraph.Parent(), firstParagraph, attentionParagraph)
 			firstParagraph.RemoveChild(firstParagraph, calloutNode)
+
+			// Remove softbreak line if there's one.
+			if firstParagraph.ChildCount() >= 1 {
+				softBreakNode, ok := firstParagraph.FirstChild().(*ast.Text)
+				if ok && softBreakNode.SoftLineBreak() {
+					firstParagraph.RemoveChild(firstParagraph, softBreakNode)
+				}
+			}
 		}
 
 		return ast.WalkContinue, nil

modules/markup/markdown/markdown_test.go

@@ -1356,4 +1356,10 @@ func TestCallout(t *testing.T) {
 	}
 
 	test(">\n0", "<blockquote>\n</blockquote>\n<p>0</p>")
+	test("> **Warning**\n> Bad stuff is brewing here", `<blockquote class="attention-header attention-warning"><p class="attention-title"><strong class="attention-warning">Warning</strong></p>
+<p>Bad stuff is brewing here</p>
+</blockquote>`)
+	test("> [!WARNING]\n> Bad stuff is brewing here", `<blockquote class="attention-header attention-warning"><p class="attention-title"><strong class="attention-warning">Warning</strong></p>
+<p>Bad stuff is brewing here</p>
+</blockquote>`)
 }