Many bugs left without severity because the no_severity_ni filters are too relaxed

[marco-c]

In particular, the query filter:

            "f16": "creation_ts",
            "o16": "greaterthan",
            "v16": lookup_nag,

means cases like these are skipped:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1943386: needinfo cancelled without setting severity, then the bug got too old
• https://bugzilla.mozilla.org/show_bug.cgi?id=1657973: bug created a long time ago and initially "enhancement", then moved to "defect"

I think we should either increase lookup_nag, or remove this filter and move it to use last activity.