Bump ethnum to placate nightly, for unified build, for oss-fuzz

[graydon]

The causal chain here is:

• oss-fuzz wants to fuzz with sanitizers turned on (a generally good idea)
• rust sanitizer builds have to be both unified builds and nightly builds (the sanitizer flag is rust-nightly-only)
• our rust build depends on ethnum, specifically version 1.5.0
• ethnum 1.5.0 has a bug in it that our pinned rust 1.95.0 doesn't mind, but rust nightly rejects; it's fixed in 1.5.3
• 1.5.3 otherwise seems to work fine and the release notes show only some optimizations and bugfixes.
• There's some theoretical potential for divergence (eg. where observable bugs in ethnum were fixed: Conversion to i128 is broken nlordell/ethnum-rs#44) but .. I don't see any way for this to actually get hit in practice: this PR is a bump to the ethnum dependency outside any of the soroban production-build deps (they all have their own pinned ones in production builds) and that dependency only exists, as far as I can tell, because stellar-xdr links in ethnum to be able to do u256/i256 ops, which .. we don't actually use anywhere in the non-soroban-host rust code (eg. the bridge helpers). There's no use of i256, u256, I256, U256, or ethnum in any rust code in our tree outside the soroban crates. So I don't think this can cause any divergence in production. But, you know, one can always miss something!

If that very narrow possible I-don't-see-how risk is still too much to swallow we can wait for a protocol boundary, but this will I think basically block oss-fuzz integration until it's resolved.

(There's another option which is to turn off the sanitizers, I guess; I am going to try that in the meantime to see if I can get it working, but I'm not sure how likely that is nor whether it's a particularly good idea longer-term.)

[Copilot]

Copilot wasn't able to review any files in this pull request.

[sisuresh]

This only affects --enable-unified-rust-unsafe-for-production builds, so seems safe to me.