github: add basic dependabot to update github actions

[ahayzen-kdab]

Note this should in theory automatically create branches to update any github action usages if they are out of date eg - uses: actions/checkout@v3 would be updated to - uses: actions/checkout@v4 etc

[ahayzen-kdab]

Right, wonder how it compares to cargo outdated as we don't have a lock file so we don't care about updating the micro version if we are only depending on the minor version in the toml