Update config.go to use new bootstrap URLs

[galt-tr]

This pull request updates the default bootstrap peer addresses in the getDefaultBootstrapPeers function to use DNSADDR records instead of hardcoded multiaddresses. This change simplifies peer discovery and makes the configuration more maintainable.

Network configuration improvements:

• Replaced hardcoded multiaddresses for main, test, and stn networks with single DNSADDR-based addresses in the getDefaultBootstrapPeers function in config.go.