buildtools: Drop Xcode earlier than 26 in clang modules build

macOS/iOS builds use Xcode 26 now.

Change-Id: I31d369986092e3ede39d287fd1438efb2778464b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6978587
Reviewed-by: Matt Stark <msta@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#1520384}
NOKEYCHECK=True
GitOrigin-RevId: 19fb0d2fdb2b6c18ccba1460d2aa7c4ea6bcdd35