lib: simplify IPv6 checks in isLoopback()

The checks for '[::1]' and '[0:0:0:0:0:0:0:1]'
in isLoopback were using startsWith,
which is unnecessary as these are canonical
loopback addresses with no valid prefixes.

Switching to strict equality improves
clarity and improves performance.

PR-URL: #59375
Reviewed-By: Tim Perry <pimterry@gmail.com>
Reviewed-By: theanarkh <theratliter@gmail.com>
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Stefan Stojanovic <stefan.stojanovic@janeasystems.com>

Author: pckrishnadas88

lib/internal/net.js

@@ -79,8 +79,8 @@ function isLoopback(host) {
   return (
     hostLower === 'localhost' ||
     hostLower.startsWith('127.') ||
-    hostLower.startsWith('[::1]') ||
-    hostLower.startsWith('[0:0:0:0:0:0:0:1]')
+    hostLower === '[::1]' ||
+    hostLower === '[0:0:0:0:0:0:0:1]'
   );
 }