Flux-core: increase minimum jansson version

[chu11]

There are a few places I'd like to use newer jansson features. Doing a grep, here's some I found in comments:

src/modules/job-manager/event.c:    /* O? support in jansson 2.8 */
src/modules/kvs/kvstxn.c:            /* jansson >= 2.7 could use json_string_length() instead */
src/modules/kvs/test/cache.c:    /* XXX - json_equal takes const in jansson > 2.10 */
src/modules/kvs/test/cache.c:    /* XXX - json_equal takes const in jansson > 2.10 */

It seems the jansson check for >= version 2.6 was written in 2016 and comment says:

    Jansson version 2.6 is a good compromise between availability
    on distros and API features.  (Except on travis-ci where version
    2.2 seems to be the available packaged version).

Dunno if these limitations still exist and we could bump up?

[garlick]

Centos7/TOSS 3 is at 2.10.
Centos8 is at 2.11.
Ubuntu 18.04 is at 2.11.
Ubuntu 20.04 is at 2.12

Seems like 2.10 would be OK at this point?

[chu11]

oh crap, i put this in the wrong repo, moving to flux-core

[chu11]

this update should also be done in flux-sched and flux-security