Bump coturn image, update chart config for configurable metrics labels

[sysvinit]

Our coturn fork previously had username labeling of Prometheus metrics patched out, however wireapp/coturn#7 reintroduced these metrics with a flag to disable them at runtime. This change updates the coturn image to include this change, and updates the configuration in the Helm chart to ensure this functionality remains disabled. This should not introduce any functional changes.

See also https://wearezeta.atlassian.net/browse/SQPIT-1012.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If this PR changes development workflow or dependencies, they have been A) automated and B) documented under docs/developer/. All efforts have been taken to minimize development setup breakage or slowdown for co-workers.
• If HTTP endpoint paths have been added or renamed, or feature configs have changed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If new config options introduced: recommended measures to be taken by on-premise instance operators.
  • If a cassandra schema migration is backwards incompatible (see also these docs), measures to be taken by on-premise instance operators are explained.
  • If a data migration (not schema migration) introduced: measures to be taken by on-premise instance operators.
  • If public end-points have been changed or added: does nginz need un upgrade?
  • If internal end-points have been added or changed: which services have to be deployed in a specific order?