Remove support for older PMD version

[adangel]

Currently pmd-regression-tester can build and execute older versions. We have several version checks to deal with the different versions (6.x and 7.x). But IMHO, this is not really necessary, as the regression tester, as it is used by pmd/pmd itself only runs against the latest version.

Therefore, we can simplify the code by removing support for older PMD versions.

Examples

• pmd-regression-tester/lib/pmdtester/builders/pmd_report_builder.rb

  Lines 200 to 203 in 15f3033

  	def should_use_long_cli_options?
  	logger.debug "PMD Version: #{@pmd_version}"
  	Semver.compare(@pmd_version, '6.41.0') >= 0
  	end

• pmd-regression-tester/lib/pmdtester/builders/pmd_report_builder.rb

  Lines 214 to 222 in 15f3033

  	def create_failonviolation_option
  	if pmd7?
  	'--no-fail-on-violation'
  	elsif should_use_long_cli_options?
  	'--fail-on-violation false'
  	else
  	'-failOnViolation false'
  	end
  	end

• pmd-regression-tester/lib/pmdtester/builders/pmd_report_builder.rb

  Lines 261 to 271 in 15f3033

  	# build command for older PMD versions
  	'./mvnw clean package ' \
  	"-s #{ResourceLocator.resource('maven-settings.xml')} " \
  	'-Pfor-dokka-maven-plugin ' \
  	'-Dmaven.test.skip=true ' \
  	'-Dmaven.javadoc.skip=true ' \
  	'-Dmaven.source.skip=true ' \
  	'-Dcheckstyle.skip=true ' \
  	'-Dpmd.skip=true ' \
  	'-T1C -B'
  	end