Dependabot: Also keep GitHub actions up-to-date
Dependabot: Also keep GitHub actions up-to-date

This adds configuratino for dependabot to also update GitHub actions.

This ensures that we have actions such as CodeQL running with relevant
versions.

Signed-off-by: Juan Antonio Osorio <e0a849f9ceef063ec3ef04542dca51583bf5b125@eu.equinix.com>

google/trillian,google/trillian