Remove support for Java 8 from Droid

[sparkhi]

We support Java 8 as a minimum version for Droid. This also means we compile with Java 8 as well during development and keep all the dependencies compliant with Java 8. As Fewer and fewer people use Java 8, and updates to it have been long gone, I think we should stop supporting Java 8 from next version onwards. We should go to (at least) Java 11 as a minimum java version as it is the next LTS version after 8.
From development and coding perspective

• Stop compiling with Java 8
• In the CI matrix, remove Java 8 (maybe keep 11, 17 and 21)
• Removal of Java 8 means on CI, we can reinstate Macos-latest instead of Macos-13
• Move to newer version of Mockito Mockito Core 5.0.0 onwards is not compatible with Java 8 #914
• There will be more plugins that can be updated.
• Update the test plan to remove Java 8
• (there may be more that we will notice)

I don't think all of this needs to happen as one big bang PR.

[steve-daly]

Just to mention that the reason we're considering this isn't due to worries about Java 8 per se, it's all the various libraries/dependencies which are becoming more and more unsupported in Java 8