Remove default "master" branch behavior

[lblackstone]

I think we should change the operator to require that either a branch or commit is specified, and get rid of the default behavior of deploying the master branch. The user is already specifying other information about the git repo in the Stack CR, so it shouldn't be an undue burden to be explicit about the desired behavior. Furthermore, both the git CLI and GitHub have moved to using main as the default branch name, so using master is no longer a good default assumption.

Affected feature

Stack CustomResource will currently deploy the latest commit from the master branch if neither branch nor commit is specified.

[elsesiy]

I agree that specifying either branch or commit via the stack CR is a reasonable expectation and is how I envisioned to use the polling feature. If anything, being explicit about which revision is tracked helps debug issues and prevent misconfigurations