closes: #12508

Remove redundant "maven." prefix from version properties in pom.xml for consistency with other properties.

Changes:

• Renamed 9 properties (removed "maven." prefix)
• Updated all 20+ references throughout the pom.xml
• Verified with mvn compile that all changes are correct

New property names:

• site.plugin.version
• spotbugs.plugin.version
• pmd.plugin.version
• jacoco.plugin.version
• checkstyle.plugin.version
• sevntu.checkstyle.plugin.version
• sevntu-checkstyle-check.checkstyle.version
• versions.plugin.version
• compiler.plugin.version

All builds pass locally.