Remove unused constant: `Vulnerability::TooManyDaysError`

The constant TooManyDaysError in the vulnerability model appears to be unused. From some git spelunking:

1. This was added in commit 5825c12f
2. It was also used in 7ff151eb
3. The resolver from 2 removed in 89009fc7
4. The usage from 1 was removed in e9567195

changed milestone to %16.7

added groupthreat insights maintenanceremoval typemaintenance labels

set weight to 1

assigned to @wandering_person

added devopsgovern sectionsec labels

mentioned in commit 0cd2f4e4

mentioned in merge request !137941 (merged)

added workflowin review label

mentioned in commit d263ada6

added workflowcomplete label and removed workflowin review label

closed