Bump chart core to v.8.15.1

[elanals]

No description provided.

[sto3psl]

The Redis logs are no errors, just log spam I need to get rid of. The actual failing test is this:

#36 src › routes › charts › {id} › assets › User can write chart asset with almost 2MB - src › routes › charts › {id} › assets › User can write chart asset with almost 2MB

      ---
        name: AssertionError
        assertion: is
        values: {"Difference:":"- 2086667\n+ 2086666"}
        at: src/routes/charts/{id}/assets.test.js:38:7
      ---

I wonder if this is a flaky test since it didn't happen in Jakubs PR.

[sto3psl]

jep. Rerunning the tests fixed it. Need to keep an eye on this.

[elanals]

Closing this because this bump was already merged from a PR that Jakub had made in parallel that I'd missed when creating this.