Set the chart's placeholder version to 0.20.0

Cosmetic: make helm-package sets the published version and appVersion
from the tag, so these two fields decide nothing (see the comment
above them). Kept in step anyway, same as 429d5fd and 6a03698, so a
tree heading for v0.20.0 doesn't say 0.19.0.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

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: 0.19.0
-appVersion: "v0.19.0"
+version: 0.20.0
+appVersion: "v0.20.0"