Clean up the fallback label code for FileIO markers

In the following code, we have some fallback code dedicated to FileIO markers:
https://github.com/firefox-devtools/profiler/blob/c87e4e9127aa1f2e423dd81712cc4f829cf24e6f/src/profile-logic/marker-schema.js#L279-L289

This doesn't seem necessary nowadays as this could be implemented as a schema.
The work here is:

1. implement in gecko the schema that does this. (or check that it's implemented)
1. remove the aforementioned fallback code
1. add migration steps to both gecko and processed profile versioning to add the data to marker schemas from older profiles.
1. possibly add tests if there's some missing for this case

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-354)

Start with src/profile-logic/marker-schema.js lines 279-289, then verify whether Gecko already defines the FileIO marker schema and locate the Gecko and processed-profile versioning migrations. Done means the schema carries the fallback data for older profiles, the fallback code is removed, and any needed tests cover migration and marker handling.

Written by the indexing model from the issue text.