FindBLAS: Find OpenMP dependency quietly

The `find_package(OpenMP)` calls added/updated by:

* commit f7f3d8987a (FindBLAS: Add dependency of OpenBLAS on OpenMP for
                     BLA_STATIC, 2020-11-10, v3.20.0-rc1~492^2)
* commit 9ef82d95d8 (FindBLAS: Fix detection of OpenMP as dependency of
                     BLA_STATIC, 2021-04-07, v3.20.1~3^2)

were missing the `QUIET` option.

Fixes: #23000