Use `silence` instead of `quietly` to silence the `CheckPending` middleware.

`Kernel.quietly` silences `STDOUT` and `STDERR`, which is useless if
the logger is writing to a file, while `AS::Logger#silence` swaps the
logger level to `ERROR`.

Related to #8820 and #8052.
Merge pull request #9087 from lucasmazza/silence-migration-checks

Use `silence` instead of `quietly` to silence the `CheckPending` middleware.