Remove dead code, unused dependencies, and stale docs

[kylebrowning]

Stacked on #246; will be retargeted to main once it merges.

• Base64.swift: 1128 → 512 lines — removes the vendored decode half and dead String helpers (production only uses encodeBytes); tests now use a small Foundation-based base64url helper.
• Deletes unused LoggingKeys; deprecates P256.Signing.PrivateKey.loadFrom(string:) (misnamed, adds nothing over init(pemRepresentation:)).
• Drops unused imports and APNSTestServer's unused Logging/Crypto/NIOSSL/NIOHTTP2 target deps; removes the swift-nio-http2 package dependency entirely (no target uses it).
• DocC: removes the Logging section documenting an API that no longer exists; fixes stale paths and copy-pasted doc comments; fixes EmptyPayload's wrong license header.
• Concurrency: APNSClientProtocol: Sendable, conditional APNSRequest: Sendable.