Remove deprecated deny config

[djc]

warning[deprecated]: this key will be removed in a future update, see https://github.com/EmbarkStudios/cargo-deny/pull/611 for details
  ┌─ /Users/djc/src/askama/deny.toml:2:1
  │
2 │ allow-osi-fsf-free = "either"
  │ ^^^^^^^^^^^^^^^^^^

warning[deprecated]: this key will be removed in a future update, see https://github.com/EmbarkStudios/cargo-deny/pull/611 for details
  ┌─ /Users/djc/src/askama/deny.toml:3:1
  │
3 │ copyleft = "warn"