Modernize Plugin Application and Build Directory

[gallardo]

Refactors build.gradle to use the modern plugins {} block and layout.buildDirectory API, replacing legacy and deprecated patterns. This PR has been split into two distinct commits to maintain a clean history.

Changes

1. Modernize Plugin Application: Migrated apply plugin and buildscript configurations to the modern plugins {} block. This enables better classloader isolation and is a prerequisite for the Gradle Configuration Cache.
2. Modernize Build Directory: Completely replaced the deprecated buildDir property with the modern layout.buildDirectory API. Adopted a cleaner syntax using layout.buildDirectory.dir("...") and layout.buildDirectory.file("...").

Justification:

• Standards: Removes deprecated property usage and adopts modern Gradle idioms.
• Maintainability: Cleaner, more declarative syntax for plugins and build paths.
• Future-proofing: Essential foundation for upcoming performance improvements like the Configuration Cache.

Verification:

• Verified that all main artifacts (JARs, dependencies) are correctly generated in the ../BuildCms directory.
• Confirmed build configuration validity via ./gradlew tasks.
• Confirmed tlddoc tasks are still correctly registered.