chore: remove usage of sha256

[TomAFrench]

Description

Problem*

Resolves

Summary*

Removing in readiness for noir-lang/noir#7477

We're just sanity checking a constant hash here which isn't worth pulling in a dependency for.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to circuit sizes

Generated at commit: e1521e307a5f58a95288fda5156c68b60d34ca4a, compared to commit: 944c159ac8e0ed14a5d381421d0dd2248bc00940

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
test_verify_sha256_pkcs1v15_2048.json	-67 ✅	-0.78%	-3,970 ✅	-16.14%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
test_verify_sha256_pkcs1v15_2048.json	8,535 (-67)	-0.78%	20,624 (-3,970)	-16.14%

[TomAFrench]

hmm, interesting that this reduces the number of opcodes as I would expect that sha256 to be optimized out entirely.

Issue captured here: noir-lang/noir#7564