Update salsa-ci.yml template and activate it by default

Most users creating new packages are likely to want to use Salsa CI to validate that the packaging does not have easily testable flaws. Thus, the salsa-ci.yml template should be active by default and not just a example file.

Also, the for most packages the file works directly as-is without modifications. The other .ex files created by dh-make are are examples because they require modifications to work, and the .ex suffix ensures they don't fail the build by introducing unnecessary/wrong definitions. For the salsa-ci.yml file there is next to no risk with using the exact template file, so there should be no need to create it with a suffix.

Additionally update the file contents to match the latest version published at https://salsa.debian.org/salsa-ci-team/pipeline/-/blob/master/recipes/salsa-ci.yml