docs: update platform-support.md to reflect latest CI and build settings

[Copilot]

What problem does this PR solve?

docs/platform-support.md was outdated — referencing Ubuntu 18.04 as Tier 1, omitting macOS AArch64 from Tier 1 despite CI fully testing it, and listing EOL distros (Debian Stretch/Buster, Arch Linux) with no official binaries.

What is changed and how it works?

Tier 1

• Ubuntu 18.04 → Ubuntu 20.04 (CI self-hosted: self-hosted-ci-ubuntu-20.04; Docker builder: focal-rust-1.92.0)
• Added macOS AArch64 (all macOS CI workflows now use macos-15, which is ARM64 Apple Silicon — unit tests + integration tests)
• Narrowed CPU SSE4.2/AVX note to x64 only

Tier 2

• Removed Ubuntu 20.04 x64 (now Tier 1), Debian Stretch (EOL 2022), Debian Buster (EOL 2024), Arch Linux (no official binary), macOS AArch64 (promoted to Tier 1)
• Renamed Ubuntu 20.04 | AArch64 → Linux (glibc) | AArch64 (cross-compiled binary is not Ubuntu-specific)
• Added CentOS EOL deprecation note
• Narrowed CPU instruction note to x64 only

Tier 3

• Any OS in Tier 1 and 2 | AArch64 | Rust → Any | Any | Rust — AArch64 ASM is now covered in Tier 1/2; Tier 3 is the catch-all for non-consensus Rust mode

Related changes

• PR to update owner/repo:
• Need to cherry-pick to the release branch

Check List

Tests

• No code

Side effects

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