chore(stdlib): remove deprecated ellipsis argument from truncate

[pront]

Summary

ellipsis has been deprecated for a long time.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.
• For new VRL functions, please also create a sibling PR in Vector to document the new function.

References

[pront]

Prepped this PR an exercise for deprecations:

• Do we need to mark this as breaking?
• Do we need changelog?

[jszwedko]

Should this be marked as a breaking change and a changelog entry included?