refactor(ci): update nightly toolchain version for udeps (#2826)

Author: boquan-fang

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ env:
   RUST_BACKTRACE: 1
   # Pin the nightly toolchain to prevent breakage.
   # This should be occasionally updated.
-  RUST_NIGHTLY_TOOLCHAIN: nightly-2025-01-31
+  RUST_NIGHTLY_TOOLCHAIN: nightly-2025-03-31
   CDN: https://dnglbrstg7yg.cloudfront.net
   # enable unstable features for testing
   S2N_UNSTABLE_CRYPTO_OPT_TX: 100