bump nginx-module-vts from v0.1.15 to v0.2.1

[fisx]

https://wearezeta.atlassian.net/browse/SER-218

upstream changelog: https://github.com/vozlt/nginx-module-vts/blob/v0.2.1/CHANGELOG.md

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[fisx]

TODO:

• Somebody (me?) should probably look at the changelog and decide if there is anything we need to be aware of.
• Make sure CI sees this and is ok with it.

[jschaul]

Should be okay (pending CI), but perhaps @flokli would like to take a quick look also.

[flokli]

Changelog looks good to me. If it builds, I guess this is fine to bump. If there's any metrics looking different now, or dashboards needing adjustments (but doesn't look like from the changelog), we can follow up.