add some guards to allow compiling with older SDK

[verokarhu]

This should unblock Homebrew from packaging it: https://github.com/Homebrew/homebrew-core/actions/runs/22052377673/job/63713257777?pr=267493#step:3:330

[GaMoCh]

I hope this resolves the issue!

Some related Homebrew Core PRs:

• Homebrew/homebrew-core#260473
  This PR attempted to update age-plugin-se to 0.2.1. In the discussion, there was a suggestion to require Tahoe for Swift 6.2+, but the author said they did not want to exclude users on older macOS versions and were instead trying to keep reduced-feature builds working there. The author also noted that macOS 14 seemed to pass, while macOS 15 was still failing for reasons they could not explain.

• Homebrew/homebrew-core#267493
  This PR attempted to update age-plugin-se to 0.2.1, but it failed to build on macOS Sequoia (15) on Apple Silicon. The failure happened because the code referenced MLKEM768 / SecureEnclave.MLKEM768, which were not available in that SDK environment.

Other relevant links:

• Issue: Implement new age1tag1.../p256tag and age1tagpq1.../p256mlkem768tag recipients #12
  This is the issue that started the discussion about support for p256mlkem768tag.

• Commit: Add support for post-quantum keys
  This is the commit that implemented that support.

• Apple documentation indicating that MLKEM768 is only available on macOS 26 (Tahoe) or newer: MLKEM768

Once the new version is accepted in Homebrew, I believe #14 should be resolved.

[GaMoCh]

A new version can probably also be released after #16 is approved.