Intent to decommission the mapper service

[Chris AtLee]

The mapper service used to provide the mappings between hg and git commits
for repositories synced by our vcs syncing service.

We have recently made the full commit id mappings available in S3 (
https://bugzilla.mozilla.org/show_bug.cgi?id=1565259), and our known
clients have switched over to using S3 instead of the mapper service.

We would like to shut off the mapper service on December 6th. This work is
being tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1599586.

Please reach out if you are still using it, and the data provided in S3
doesn't work for your use case.

Cheers,
Chris