Allow Jackson to escape new line chars when BOMR adds issues

Remove the escaping logic when building the issue body so that Jackson
can perform the actual escaping. Prior to this commit, the message
body was double escaped.

Closes gh-43479

Author: philwebb

buildSrc/src/main/java/org/springframework/boot/build/bom/bomr/MoveToSnapshots.java

@@ -93,7 +93,7 @@ protected String issueBody(Upgrade upgrade, Issue existingUpgrade) {
 		if (existingUpgrade != null) {
 			lines.add("Supersedes #" + existingUpgrade.getNumber());
 		}
-		return String.join("\\r\\n\\r\\n", lines);
+		return String.join("\n\n", lines);
 	}
 
 	@Override

buildSrc/src/main/java/org/springframework/boot/build/bom/bomr/UpgradeBom.java

@@ -88,7 +88,7 @@ protected String issueBody(Upgrade upgrade, Issue existingUpgrade) {
 		if (existingUpgrade != null) {
 			lines.add("Supersedes #" + existingUpgrade.getNumber());
 		}
-		return String.join("\\r\\n\\r\\n", lines);
+		return String.join("\n\n", lines);
 	}
 
 }