Switch to PEP 639 license metadata

[NMertsch]

Summary

Switch license metadata format in pyproject.toml from PEP 621 to PEP 639.

Before:

[project]
licence-file = "LICENSE"

After, license from SPDX License List:

[project]
license = "BSD-3-Clause"  # or other SPDX license identifier
license-files = ["LICENSE"]

After, proprietary license:

[project]
license = "LicenseRef-Proprietary"
license-files = ["LICENSE"]

Details

• Change: Remove the license option "Other", to be re-introduced via [not-yet-created issue]
• Requirement: This PR needs to be accompanied by a PR in https://github.com/beeware/briefcase to update briefcase dev|create|convert

Partial fix for beeware/beeware#2146.

PR Checklist:

• All new features have been tested
• All new features have been documented (to be done in the briefcase PR)
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct