several improvements and fixes after switching WTO to be dependent on DWO

This PR provides the following changes:

fix controller image and make github workflow build the next tag;
return back target for uninstalling WTO 1.2;
~~disable an ability to install operator in All namespace mode, since it's redundant in our case at this point, and it's the time we can migrate to newer preferable way;~~ moved to https://github.com/redhat-developer/web-terminal-operator/pull/82
add pr check to verify go sources and controller images;
upgrade controller to go 1.15 and use newer way to install go in ubi image suggested in https://github.com/eclipse/che/issues/19762;

redhat-developer / web-terminal-operator