Avoid iOS 16 URL parsing crash in stringWithUserAndPasswordStripped

[o-nnerb]

Summary

• URL.user()/.password() (the percent-encoded accessors) share internal parsing code with URL.host(percentEncoded:) on iOS 16, and calling them there can crash inside host parsing even though host is never touched here (same crash signature as https://forums.swift.org/t/70452).
• Gates the fast path on iOS 17 instead of iOS 16 (the version these APIs were actually introduced in), so iOS 16 falls back to the legacy, non-percent-encoded user/password properties, which don't hit the bug.
• Previously merged into beta via Avoid iOS 16 URL parsing crash in stringWithUserAndPasswordStripped #4; this re-targets the same fix at main since main now tracks upstream directly rather than carrying this fork's cumulative history (see the release integration branch for where this and the fork's other pending work live together).

Test plan

• swift build clean.
• No test coverage added — this is a crash-avoidance gate with no observable behavior difference on iOS 17+; a regression test would need to run on iOS 16 specifically to exercise the crash path.

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