Replaces static mutable references in entry

[brooksprumo]

Problem

New clippy lints when upgrading to rust 1.84.0.

Some are related to creating references to mutable statics. Here's the new lint for entry:

warning: creating a shared reference to mutable static is discouraged
  --> entry/src/entry.rs:83:14
   |
83 |     unsafe { API.as_ref() }
   |              ^^^^^^^^^^^^ shared reference to mutable static
   |
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html>
   = note: shared references to mutable statics are dangerous; it's undefined behavior if the static is mutated or if a mutable reference is created for it while the shared reference lives
   = note: `#[warn(static_mut_refs)]` on by default

Summary of Changes

Replace Once with OnceLock for the API, as it is the preferred way to do Once+data.

Partially fixes #4380

[alexpyattaev]

Lgtm

[brooksprumo]

@bw-solana Can I get your review here as well, please? I want to make sure I haven't missed anything. Feel free to tag in others to review as well.

[bw-solana]

LGTM! Left one minor suggestion

[bw-solana]

🎉

[bw-solana]

LGTM