Update gradle & ui

Description

Updates Gradle and the UI package.

Changes

• Updated Gradle from 5.4.1 to 6.6.1
• Updated Spotbugs
• Updated Checkstyle
• Updated UI

Additions

None.

Test and Review

To be filled in by the reviewers

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date