Update the developer mailing list address.

The developer mailing list was migrated to Google Groups in February 2024
as announced in https://groups.google.com/g/bitcoindev/c/aewBuV6k-LI.

The archives maintained by the Linux Foundation stopped updating in December
2024. Thus, we point to the new archive maintained by gnusha.org.

The codebase refers to old discussions linked to the Linux Foundation archives.
Since all links are still active to this date, we keep them as they are.

See #29782.

CONTRIBUTING.md

@@ -66,9 +66,10 @@ Discussion about codebase improvements happens in GitHub issues and pull
 requests.
 
 The developer
-[mailing list](https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev)
+[mailing list](https://groups.google.com/g/bitcoindev)
 should be used to discuss complicated or controversial consensus or P2P protocol changes before working on
 a patch set.
+Archives can be found on [https://gnusha.org/pi/bitcoindev/](https://gnusha.org/pi/bitcoindev/).
 
 
 Contributor Workflow

doc/dnsseed-policy.md

@@ -44,7 +44,7 @@ related to the DNS seed operation.
 If these expectations cannot be satisfied the operator should
 discontinue providing services and contact the active Bitcoin
 Core development team as well as posting on
-[bitcoin-dev](https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev).
+[bitcoin-dev](https://groups.google.com/g/bitcoindev).
 
 Behavior outside of these expectations may be reasonable in some
 situations but should be discussed in public in advance.