MEI: Adjust the weight for untriaged bugs to 3

[jensstutte]

Based on this doc.

Currently untriaged defects have a weight of 5. While it is true that triaging should have a high priority, it is unlikely that many of those defects will end up being S1 or S2.

Furthermore we already used 3 for old severity values, which are most likely to be considered equally untriaged (and have become rare anyways, it seems).

Recap of the resulting weights:

S1: 8
S2: 5
untriaged/old severity: 3
S3: 2
S4: 1

[marco-c]

and have become rare anyways, it seems

There should be basically 0, as we migrated the old values to the new ones recently (there might be 1 or 2 because of intermittent bugs that were closed at the time we did the migration and reopened after we did the migration).