s/masterminds/mattermost for squirrel (#20162)

Our fork of https://github.com/Masterminds/squirrel at https://github.com/mattermost/squirrel (previously https://github.com/lieut-data/squirrel) isn't compatible with the upstream project, and causes issues for repositories importing mattermost-server and using squirrel.

Sidestep this by adopting the `mattermost/squirrel` fork explicitly.  (Downstream repos can either switch, or keep using the upstream version from `Masterminds`.)

go.sum

@@ -988,6 +988,8 @@ github.com/mattermost/morph v0.0.0-20220401091636-39f834798da8 h1:gwliVjCTqAC01m
 github.com/mattermost/morph v0.0.0-20220401091636-39f834798da8/go.mod h1:jxM3g1bx+k2Thz7jofcHguBS8TZn5Pc+o5MGmORObhw=
 github.com/mattermost/rsc v0.0.0-20160330161541-bbaefb05eaa0 h1:G9tL6JXRBMzjuD1kkBtcnd42kUiT6QDwxfFYu7adM6o=
 github.com/mattermost/rsc v0.0.0-20160330161541-bbaefb05eaa0/go.mod h1:nV5bfVpT//+B1RPD2JvRnxbkLmJEYXmRaaVl15fsXjs=
+github.com/mattermost/squirrel v0.2.0 h1:8ZWeyf+MWQ2cL7hu9REZgLtz2IJi51qqZEovI3T3TT8=
+github.com/mattermost/squirrel v0.2.0/go.mod h1:NPPtk+CdpWre4GxMGoOpzEVFVc0ZoEFyJBZGCtn9nSU=
 github.com/mattn/go-colorable v0.0.9/go.mod h1:9vuHe8Xs5qXnSaW/c/ABM9alt+Vo+STaOChaDxuIBZU=
 github.com/mattn/go-colorable v0.1.1/go.mod h1:FuOcm+DKB9mbwrcAfNl7/TZVBZ6rcnceauSikq3lYCQ=
 github.com/mattn/go-colorable v0.1.2/go.mod h1:U0ppj6V5qS13XJ6of8GYAs25YV2eR4EVcfRqFIhoBtE=