Replace Proprietary and Other with SPDX LicenseRef identifiers

[Georgefifth]

Problem:

• Paired with Emit valid SPDX identifiers for the Proprietary and Other license options briefcase#3021: the new-project wizard will pass LicenseRef-Proprietary and LicenseRef-Other as license values, because Proprietary and Other are not valid SPDX expressions under PEP 639. The template currently rejects those values.

Solution:

• Replace the Proprietary and Other choices with LicenseRef-Proprietary and LicenseRef-Other.
• Render the existing proprietary LICENSE text for LicenseRef-Proprietary (as before); Other had no LICENSE template branch, so behaviour is unchanged.

Result:

• briefcase new -Q license=LicenseRef-Proprietary generates a project with a valid SPDX license value and the proprietary LICENSE text; verified end-to-end against the paired briefcase PR.
• Full test suite: 12 passed.

Refs beeware/briefcase#3016

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Hermes Agent (Qwen3.8-Max) — changes reviewed and verified end-to-end by the operator

[freakboy3742]

Thanks for the PR, but you've massively overengineered this, and slightly missed the point.

The original issue is that Proprietary isn't a valid value for a license - it needs to be an SPDX identifier.

No unit test is needed for this change - just a change to the template. The test that is presented here doesn't actually validate anything.

[freakboy3742]

As noted on the Briefcase PR, we should also be handling "Other"

Suggested change

	"Other"
	"LicenseRef-Other"

[Georgefifth]

Addressed: the test is removed, and "Other" is now replaced with LicenseRef-Other alongside LicenseRef-Proprietary. I also dropped the backward-compat dual values — briefcase releases pin a template hash, so there is no real-world mixing of old and new values, and the extra choices were just noise.

[freakboy3742]

Thanks for those updates!