Allow setting level 3 task-expires-after in the graph configuration

[gabrielBusta]

The way the graph config could look like:

task-expires-after:
    by-level:
        1: 90 days
        3: 1 year

We could set a single default for all levels:

task-expires-after: 1 year

[gabrielBusta]

I looked into this a bit. How could we deal with setting different values for the docker image kind vs. the fetch kind vs. the rest? Docker image defaults to 1 year on level 3, but fetches default to 1000 years.