set helm chart appVersion while release

[ikeeip]

What this PR does / why we need it:

Release script tools/release_prepare.sh doesn't updates appVersion in helm charts while release. This MR fixes this.

before:

$ helm search loki
NAME            	CHART VERSION	APP VERSION	DESCRIPTION
loki/loki       	0.13.0       	0.0.1      	Loki: like Prometheus, but for logs.
loki/loki-stack 	0.14.0       	0.0.1      	Loki: like Prometheus, but for logs.
...

after:

$ helm search loki
NAME            	CHART VERSION	APP VERSION	DESCRIPTION
local/loki      	0.14.0       	v0.2.0     	Loki: like Prometheus, but for logs.
local/loki-stack	0.15.0       	v0.2.0     	Loki: like Prometheus, but for logs.
...

Which issue(s) this PR fixes:
none

Special notes for your reviewer:

I have run fixed tools/release_prepare.sh locally and this is why local/loki package shown in the example.

Checklist
not needed

[CLAassistant]

All committers have signed the CLA.

[slim-bean]

Good catch LGTM!