add task signatures to koji.task.LEGACY_SIGNATURES

[tkopecek]

koji policies can use src/target in policy checks if mapping for these exists in the hub.

Maintainers will complete the following section

• [ ] Commit messages are descriptive enough
• [ ] Code coverage from testing does not decrease and new code is covered
• [ ] JSON/YAML configuration changes are updated in the relevant schema
• [ ] Pull request has a link to an osbs-docs PR for user documentation updates

[rcerven]

@tkopecek please add signed-off to the commit

[tkopecek]

updated