Update .github/dependabot to point at all actions

[smoser]

The directories key means less repeating ourselves. Note the setup-* should match all setup- entries.

[smoser]

I ran this against fork https://github.com/smoser/cgrdev-actions/actions/runs/18495501420/job/52698656119 and it did run but could not make a pr.. i guess i didn't configure that.

At the bottom it printed this:

 +-----------------------------------------------------------------+
|               Changes to Dependabot Pull Requests               |
+---------+-------------------------------------------------------+
| created | sigstore/cosign-installer ( from 3.8.2 to 3.10.0 )    |
| created | chainguard-dev/setup-chainctl ( from 0.1.0 to 0.4.0 ) |
| created | azure/setup-helm ( from 2.1 to 3.5 )                  |
+---------+-------------------------------------------------------+