Update gradle and plugin dependencies

[BClark09]

• redline plugin 1.2.6 to 1.2.10 and brought forward my own changes for signing
• bumped ospackage plugin 4.5.0 to 8.1.0 and brought forward my own changes for signing
• Gradle 4.0.1 to Gradle 6.1.1

Fixes a few build issues, nothing noticible by the end user.

The project can now be built with Java 11+

Signed-off-by: Ben Clark ben@benjyc.uk