Add constants for common licenses, update to Gradle 7.1, and fix check for tomcat plugin

[labkey-susanh]

Rationale

As we begin to use the BuildUtils.addExternalDependency in order for Gradle to produce the jars.txt files for us, we can use some constants for common licenses and URLs.

The check for the existence of the tomcat plugin was not quite right, causing cleanDeploy and cleanBuild to no longer do a shutdown of tomcat, leaving the dev environment in an unhappy state.

Related Pull Requests

#123

Changes

• Add some constants helpful for declaring external dependencies
• Fix check for tomcat plugin so cleanBuild and cleanDeploy will stop tomcat if available
• Update to gradle 7.1