build: pin ethnum to 1.5.3

[shitcodebykaushik]

Description of Changes

Pins the workspace ethnum dependency to exactly 1.5.3. Versions before 1.5.3 rely on the representation of an error type that changed in newer Rust toolchains, which can break downstream users even though SpacetimeDB's pinned toolchain is unaffected.

The lockfile already resolved ethnum 1.5.3, so no lockfile update is required.

Fixes #5512.

API and ABI breaking changes

None.

Expected complexity level and risk

1. This is a one-line workspace dependency constraint change to the version already present in Cargo.lock.

Testing

• cargo metadata --locked --no-deps --format-version 1
• cargo check --locked -p spacetimedb-sats -p spacetimedb-expr -p spacetimedb-table -p spacetimedb-memory-usage
• cargo test --locked -p spacetimedb-sats -p spacetimedb-expr -p spacetimedb-table -p spacetimedb-memory-usage --lib
• git diff --check

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