refactor: Change main thread name to xrpld-main (#6336)

This change builds on the thread-renaming PR (#6212), by renaming the main thread name to reduce ambiguity in performance monitoring tools.

Author: vlntb

src/xrpld/app/main/Main.cpp

@@ -323,7 +323,7 @@ run(int argc, char** argv)
 {
     using namespace std;
 
-    beast::setCurrentThreadName("main");
+    beast::setCurrentThreadName("xrpld-main");
 
     po::variables_map vm;