Chore: Add gradle plugin to generate NOTICE.md automatically

[SammyIsConfused]

This came up as a concern during #63 - so whilst Renovate works properly for updating dependencies, the NOTICE.md file does not change when dependencies do, it has to be maintained manually.

Generally, NOTICE files are automatically generated - thankfully since this is a gradle project there are definitely lots examples of plugins we could use to achieve this:

• https://github.com/jk1/Gradle-License-Report
• https://github.com/jaredsburrows/gradle-license-plugin
• https://github.com/hierynomus/license-gradle-plugin
• https://github.com/eskatos/honker-gradle

It would be good to implement one of these so that NOTICE.md does not need to be maintained manually.