Fixes n0mer/gradle-git-properties#239: type projectVersion as Property<String>

[rivancic]

• Change projectVersion from Property<Object> to Property<String>
• Initialize via Provider and toString() to ensure a String value
• Remove redundant public modifiers (Groovy defaults to public)
• Update getters to return correctly typed properties

[rivancic]

I’m not fully sure whether narrowing projectVersion from Property<Object> to Property<String> could limit any existing usage where a non-String value (e.g., GString or custom type) was being provided. In practice Gradle treats version as stringy and we explicitly coerce to String via a Provider, but I’d appreciate feedback from maintainers/users if you know of cases that rely on the broader Object typing.

I tested this locally with the fr.brouillard.oss.gradle.jgitver plugin (I know not really maintained 🙃 ) and the project version was resolved correctly with this change. It fixed the behavior described in #239.