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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 52.56%. Comparing base (d263722) to head (f5924f3).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #630 +/- ## =========================================== + Coverage 52.37% 52.56% +0.18% =========================================== Files 14 14 Lines 1600 1600 =========================================== + Hits 838 841 +3 + Misses 706 702 -4 - Partials 56 57 +1 ``` | [Flag](https://app.codecov.io/gh/flashbots/mev-boost/pull/630/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/flashbots/mev-boost/pull/630/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `52.56% <100.00%> (+0.18%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more.

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