dns: revise quadratic regex in setServers

Problem:
The IPv6 regex was quadratic.
On long malicious input the event loop could block.

The security team did not deem it a security risk,
but said a PR was welcome.

Solution:
Revise the regex to a linear-complexity version.

Tests:
I added REDOS tests to the "oddities" section.

Fixes: #20443

lib/dns.js

@@ -289,7 +289,7 @@ function setServers(servers) {
   // servers cares won't have any servers available for resolution
   const orig = this._handle.getServers();
   const newSet = [];
-  const IPv6RE = /\[(.*)\]/;
+  const IPv6RE = /^\[([^[\]]*)\]/;
   const addrSplitRE = /(^.+?)(?::(\d+))?$/;
 
   servers.forEach((serv) => {

test/parallel/test-dns.js

@@ -68,7 +68,10 @@ assert(existing.length > 0);
     ' ',
     '\n',
     '\0',
-    '1'.repeat(3 * 4)
+    '1'.repeat(3 * 4),
+    ':'.repeat(100000),
+    '['.repeat(100000),
+    '['.repeat(100000) + ']'.repeat(100000) + 'a'
   ];
   invalidServers.forEach((serv) => {
     assert.throws(() => dns.setServers([serv]), /TypeError.*ERR_INVALID_IP_ADDRESS/, `Unexpected error thrown for ${serv}`);