docs(common): convert licenses to SPDX format

[david-allison]

Purpose / Description

AnkiDroid is standardizing licenses to SPDX.

Fixes

• Part of issue Move to SPDX for file-level license headers (mono-license GPL/LGPL only) #20954

Approach

I removed my copyright lines from the files as-per docs/contributing/copyright-headers.md

I added a license header to build.gradle.kts

How Has This Been Tested?

This change was not performed using LLMs and I have verified that it is correct

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner