Update Gradle to 7.3. in order to profit from the performance improvement and to prepare for dependency lockfiles. Due to the upgrade to 7.x we had to use a license plugin which supports Gradle 7.
Add an experimental high contrast color theme to SCM-Manager. The high contrast mode uses a dark background and color with a high contrast for a better accessibility.
The change adds the theme to ui-styles and theme switcher to the storybook of ui-components.
