refactor(semaphore): remove acquire-and-forget helpers

[tisonkun]

Summary

Remove these redundant convenience methods for 0.7:

• Semaphore::try_acquire_and_forget
• Semaphore::acquire_and_forget
• Semaphore::try_acquire_owned_and_forget
• Semaphore::acquire_owned_and_forget

Permit-level SemaphorePermit::forget and OwnedSemaphorePermit::forget remain unchanged. The changelog documents the migration.

Rationale

These helpers were introduced in #33 together with the public Semaphore::release_all API for an uncoupled acquire/release workflow. #34 removed release_all because it was a footgun, leaving four helpers that only compose existing operations.

The same behavior remains explicit through permit guards:

semaphore.acquire(n).await.forget();

if let Some(permit) = semaphore.try_acquire(n) {
    permit.forget();
}

This operation still uses the normal fair acquisition queue and remains cancellable while waiting. It is intentionally distinct from the immediate bounded drain_permits and exact reduce_permits operations clarified in #174. Expressing it as acquisition followed by permit-level forget preserves that distinction without four extra public methods.

Testing

• cargo x lint
• cargo x check
• cargo x test --no-capture
• cargo test --workspace --all-features