ci: store image artifacts in separate paths

[Ressetkk]

/kind chore
/area ci

To unblock easy deployment of #1150 places where main and release images are put need to be separated.

Because we push images created by post-api-gateway-manager-build and rel-api-gateway-manager-build to the same directory with the same build tag format europe-docker.pkg.dev/kyma-project/prod/api-gateway-manager:vDATE-ShortSHA. That results in mix-matching the images used in main and in production. It also complicates developing simple automations around our images if we wanted to keep implementations of automated tasks separated.
My suggestion is to separate main and release images with subpath:

• General idea: europe-docker.pkg.dev/kyma-project/prod/{REPOSITORY}/{TYPE}/{NAME}
• main images build to europe-docker.pkg.dev/kyma-project/prod/api-gateway/main/api-gateway-manager
• release images build to europe-docker.pkg.dev/kyma-project/prod/api-gateway/releases/api-gateway-manager

This will not affect the way how our images are signed or validated, as validation is configured against europe-docker.pkg.dev/kyma-project/prod