Set the chart's placeholder version to 0.15.0

Cosmetic, as in 4c85e76 and 041e159. `make helm-package` passes --version
and --app-version from the tag, so neither field decides anything about
what release.yaml publishes.

Done anyway because a tree heading for v0.15.0 that still says 0.14.0
tells a reader something false. appVersion keeps the v, per
APPVERSION_PREFIX in .release.conf.

charts/terdut-server/Chart.yaml

@@ -15,5 +15,5 @@ type: application
 # appVersion and image.tag in values.yaml no longer agree, and that is not an oversight:
 # image.tag stays "latest", which is what a local install actually pulls. appVersion is
 # metadata and drives nothing.
-version: 0.14.0
-appVersion: "v0.14.0"
+version: 0.15.0
+appVersion: "v0.15.0"