Downgrade to Golang 1.15

[timuthy]

How to categorize this PR?

/area delivery
/kind regression
/priority normal

What this PR does / why we need it:
Go 1.16 comes with import restrictions which cause the following errors:

github.com/gardener/terraformer/hack imports
	github.com/gardener/gardener/.github: malformed import path "github.com/gardener/gardener/.github": leading dot in path element
github.com/gardener/terraformer/hack imports
	github.com/gardener/gardener/.github/ISSUE_TEMPLATE: malformed import path "github.com/gardener/gardener/.github/ISSUE_TEMPLATE": leading dot in path element
github.com/gardener/terraformer/hack imports
	github.com/gardener/gardener/hack/.ci: malformed import path "github.com/gardener/gardener/hack/.ci": leading dot in path element
make[1]: *** [/go/src/github.com/gardener/terraformer/Makefile:151: revendor] Error 1
make[1]: Leaving directory '/go/src/github.com/gardener/terraformer'
make: *** [Makefile:163: check-generate] Error 1

We gave feedback in golang/go#43985 and until a decision is made, we downgrade to 1.15 again.

Release note:

NONE

[timuthy]

I suppose the verification will continue to fail because of the .pipeline_definition on the master branch?! 👀

[timebertt]

Thanks!
/lgtm

I suppose the verification will continue to fail because of the .pipeline_definition on the master branch?! 👀

Yes, unfortunately the pipeline definitions from the PR branch is not merged into the one from the base branch...
So we have to force-merge again. @rfranzke can you help with that?