Activate Spotless via Maven property rather than file

[basil]

Thanks to the many plugin maintainers who have adopted the new configuration introduced in #733. This has allowed me to clean up the way this configuration is activated by lifting it out of a custom profile into the main section of the build. To keep it off by default I am defining <spotless.check.skip>true</spotless.check.skip> by default. Plugins can override this property to <spotless.check.skip>false</spotless.check.skip> to enable Spotless. The documentation now recommends this over the file-based activation method. The file-based activation method still works as a compatibility bridge but just sets this property. After this PR is merged, I will go through any plugins using the file-based activation method and convert them to the property method, then finally remove the file-based activation from this POM.

The only risk here is to the handful of plugins that are still defining their own configuration:

• Use upstream Spotless configuration github-branch-source-plugin#684 filed to use the standard configuration
• Enable Spotless for code formatting git-plugin#1435 filed to use the standard configuration
• Use upstream Spotless configuration gitlab-plugin#1449 to use the standard configuration
• https://github.com/jenkinsci/allure-plugin is in need of modernization in Modernize plugin allure-plugin#325 before this can be fixed
• https://github.com/jenkinsci/artifact-repository-parameter-plugin appears unmaintained
• https://github.com/jenkinsci/gcp-secrets-manager-credentials-provider-plugin appears unmaintained
• https://github.com/jenkinsci/mattermost-plugin appears unmaintained

Since I have filed PRs to update most of these to use the standard configuration I am not worried. If for some reason the maintainers refuse to use the standard configuration then they will have to figure out how to harmonize their configuration with mine. An example of how to do this is in jenkinsci/additional-metrics-plugin#23 where I blended in the maintainer's preference for a license header check.

As part of this PR I have also enabled Spotless for this repository's test suite and formatted any test plugins to enhance test coverage.

[basil]

If any of the above maintainers encounter an issue with this PR, they can either remove their non-standard Spotless configuration or contact me for help with harmonizing the two configurations if necessary. In many cases I have already reached out with no response.