Updated SPDX identifiers from CC0-1.0 to MIT across multiple files, including `.github`, `CONTRIBUTING.md`, `README.md`, and more. Deleted the `LICENSES/CC0-1.0.txt` file as it is no longer relevant.
- Also import the original BSD-3-Clause.txt license from the Go team into the LICENSES directory
- Further on, license headers should hold "The go-mail Authors" instead of my name. Did this already for the MIT license.
# SUMMARY
* Bad licenses:
* Deprecated licenses:
* Licenses without file extension:
* Missing licenses:
* Unused licenses:
* Used licenses: CC0-1.0, MIT
* Read errors: 0
* Files with copyright information: 45 / 45
* Files with license information: 45 / 45
Congratulations! Your project is compliant with version 3.0 of the REUSE Specification :-)