Temporarily disable backport linter

This issue is to flag that {lintr} throws confusing warnings from the backport linter when encountering a dependency on R >= 2.10, which causes the lint-changed-files workflow to fail.

This is temporarily fixed by

- [x] Disabling the backport linter in `.lintr`
- [ ] Re-enabling backport linter in `.lintr` once {lintr} issue is fixed

Read `.lintr` and inspect the `lint-changed-files` workflow to understand the disabled backport linter. Check whether the {lintr} dependency warning is fixed; done means re-enabling the linter in `.lintr` without causing the workflow to fail.

Written by the indexing model from the issue text.