[EoL detector] Add detection for Ubuntu Legacy

Ubuntu offers 3 level of maintenance: standard, pro and legacy.
The end of life detector currently detects standard and pro, but not legacy, so it's possible it returns false positives as legacy has 2 years of extra support.
Not sure what files we could check here to detect such configuration as I don't have access to a machine with such support enabled.

Path: detector/endoflife/linuxdistro/ubuntu.go

Start in detector/endoflife/linuxdistro/ubuntu.go and investigate how Ubuntu standard and pro support are currently detected. Determine what configuration identifies Legacy support, then verify that its extra support period no longer produces a false positive; the issue does not name a test file.

Written by the indexing model from the issue text.