Make UnknownSafeOrFinalizedBlock error string lower case

[joshieDo]

Describe the change

#7552 introduces a capital letter on an error message when querying reth for the finalized block

reth/crates/rpc/rpc/src/eth/error.rs

Lines 40 to 48 in fd51d36

	/// Thrown when querying for `finalized` or `safe` block before the merge transition is
	/// finalized, <https://github.com/ethereum/execution-apis/blob/6d17705a875e52c26826124c2a8a15ed542aeca2/src/schemas/block.yaml#L109>
	///
	/// op-node uses case sensitive string comparison to parse this error:
	/// <https://github.com/ethereum-optimism/optimism/blob/0913776869f6cb2c1218497463d7377cf4de16de/op-service/sources/l2_client.go#L105>
	///
	/// Temporary, until a version of <https://github.com/ethereum-optimism/optimism/pull/10071> is pushed through that doesn't require this to figure out the EL sync status.
	#[error("Unknown block")]
	UnknownSafeOrFinalizedBlock,

Reason being that op-node is case sensitive on parsing the error during EL sync mode initialization:
https://github.com/ethereum-optimism/optimism/blob/0913776869f6cb2c1218497463d7377cf4de16de/op-service/sources/l2_client.go#L105

This issue is blocked until that changes, or the EL sync intialization flow changes.

Additional context

No response