Use go-eth2-client's IsInfinity() to check relay pubkey

[jtraglia]

📝 Summary

Just fixing a few things I noticed while reviewing.

• Use go-eth2-client's IsInfinity function to check the relay pubkey.
• Provide valid pubkey to TestParseRelaysURLs & TestHexToPublicKey tests.
• Add additional test cases to TestParseRelaysURLs.
• Uncomment "invalid hexadecimal input" tests, might as well.
• Update go-boost-utils to v1.8.1 and run go mod tidy.

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✅ I have run these commands

• [x] make lint
• [x] make test-race
• [x] go mod tidy