info: Reduce safety limit to 50 ms

After reducing the DGA query separation to 100 ms, the DGA safety kick in and
limits the DGA frequency. This patch changes the safety limit to 50 ms (20 Hz).

Target: trunk
Request: 2.11
Request: 2.10
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7540/