Bump github.com/attestantio/go-eth2-client from 0.19.8 to 0.21.4

[dependabot]

Bumps github.com/attestantio/go-eth2-client from 0.19.8 to 0.21.4.

Changelog

Sourced from github.com/attestantio/go-eth2-client's changelog.

0.21.4:

• additional nil checks
• allow non-mainnet configurations

0.21.3:

• relax requirement for proposals to use the graffiti we request

0.21.2:

• fuzz testing fixes

0.21.1:

• fix potential crash when unmarshaling Gwei values
• add WithReducedMemoryUsage() option for http service
• more consistent tracing attributes and codes

0.21.0:

• use v3 of the endpoint to obtain proposals
• add bounds checking for ValidatorState

0.20.0:

• allow delayed start of client, enabling the service even if the underlying beacon node is not ready
• add IsActive() and IsSynced() methods to understand the status of the service
• update multi clients to be aware of delayed start, only using clients that are synced
• use standard errors for common function issues
• add ProposerIndex() to VersionedSignedProposal
• add name to multi clients to differentiate multiple instances
• fully parse provided client URLs, allowing pass through of username, password, etc.

0.19.10:

• add proposer_slashing and attester_slashing events
• add bls_to_execution_change event

Commits

• e02b07f Updates for release.
• 70f5e25 Update dependencies.
• 56dfbe8 Additional linting.
• 9b90d7b Merge pull request #138 from jtraglia/add-missing-nil-checks
• f50dcb2 Add missing nil checks to two Versioned structs
• 2730975 Merge pull request #123 from pk910/pk910/dynamic-ssz
• 902e2d4 Revert use of bad yaml library version.
• 3e81eac Merge pull request #135 from jtraglia/epoch-json-marshal-funcs
• ed02a7f Update types.go
• 2fc06f1 Update types.go
• Additional commits viewable in compare view

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