Fix integration tests to use correct Maven version instead of hardcoded 2.1-SNAPSHOT (#2476)

Switch default behavior from using build Maven installation to testing
the Maven distribution generated by the current build. This eliminates
the need for manual version updates during releases.

- Remove hardcoded maven-version property
- Align its/ and core-it-suite/ versions with parent project
- Keep core-it-support/ artifacts at stable 2.1-SNAPSHOT version
- Add maven-from-build profile to preserve old behavior
- Update documentation for new usage patterns

Default: mvn clean test -Prun-its (tests built distribution)
Old behavior: mvn clean test -Prun-its,maven-from-build

Author: gnodet