VS: Do not apply '/external:W*' flag table mapping on VS < 16.10

Since commit 9054cd05 (VS: Add flag table entries for '/external:W*'
flags in VS 16.10, 2021-05-28, v3.20.4~10^2) we map flags to the
`ExternalWarningLevel` element.  VS 16.9 does not support that element,
but its `cl` compiler does support the `/external:W*` flags.  Filter out
the flag table entry on older VS versions.

Fixes: #22308