Fix image branch naming assumptions in autodevops.sh

What does this MR do?

Fix image branch naming assumptions in autodevops.sh

Due to the changes introduced by gitlab-org/release-tools!2362
in populating a leading `v` into the appVersion - this had a
cascade effect in image_branch having a leading `v` - causing
the image tag for the stable branch to erroneously be set to
`vX-Y-stable`

This removes the leading `v` coming from appVersion when used
to match an image with the correct stable branch name.

Changelog: fixed
Signed-off-by: Jason Young <jyoung@gitlab.com>

Related issues

Closes #4747 (closed)

Checklist

See Definition of done.

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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
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• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.