Maven 3.9.x should use Maven 3.1 validation in strict mode

[gnodet]

Changes VALIDATION_LEVEL_STRICT from VALIDATION_LEVEL_MAVEN_3_0 (30) to VALIDATION_LEVEL_MAVEN_3_1 (31).

In strict mode (the default for project builds), the getSeverity(request, errorThreshold) method returns ERROR when validationLevel >= threshold and WARNING otherwise. By raising the strict level from 3.0 to 3.1, the following checks are promoted from warnings to errors:

• Duplicate plugin declarations in build/plugins
• Banned characters in version strings (e.g. backslash, expression fragments)
• Unsupported snapshot version formats (non-standard SNAPSHOT qualifiers)
• Banned characters in repository IDs
• Reserved repository ID local

Additional warnings are now emitted for:

• system scope dependency deprecation
• Hard-coded system paths in systemPath
• ${basedir}-relative system paths

The test resource duplicate-plugins-merged-pom.xml was updated to move the duplicate plugin into a profile (avoiding the raw validation error while still testing plugin merge behavior).

Unit tests updated to explicitly use VALIDATION_LEVEL_MAVEN_3_0 where the test exercises pre-3.1 behavior, and to use the default (now 3.1) level where the stricter validation is expected.

Companion IT PR: apache/maven-integration-testing#441

[cstamas]

Let's see CI, as it may have some quirks...

[cstamas]

Oups, not yet, these are IT failures...

[gnodet]

Companion IT PR: apache/maven-integration-testing#441