os_version_num() fails for modern Linux version strings .

[clemej]

Describe the bug
I tried to create a custom app plan, and one stipulation is for the user to have a kernel > 5.0.0. But when I set os_min_version, it didn't work.. turning on debugging, I see this in the logs:

[version] plan_class_spec: Can't determine numerical OS version 'Ubuntu 20.10 [5.10.0-051000rc3-generic|libc 2.32 (Ubuntu GLIBC 2.32-0ubuntu3)]'

The code in question is essentially if (sscanf(string, "%u.%u.%u", &a, &b, &c) == 3), and sure enough the above string won't parse to that. Maybe there's meant to be a search for '[' before ?

https://github.com/BOINC/boinc/blob/master/sched/plan_class_spec.cpp#L51

Steps To Reproduce

1. make a custom app plan that sets a min_os_version , turn on version debugging. See error in logs.

Expected behavior
The os_min_version number is set correctly on Linux.