[BUG][GRADLE] Gradle plugin does not properly support inputSpecRootDirectory

[Luckl]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

See #15483. The PR fixing this issue marks the configuration option with @InputFile which does not work for a directory. This causes the following exception to be thrown:

A problem was found with the configuration of task ':openApiGenerate' (type 'GenerateTask').
  - In plugin 'org.openapi.generator' type 'org.openapitools.generator.gradle.plugin.tasks.GenerateTask' property 'inputSpecRootDirectory' file 'C:\xx\xx\xx\contracts' is not a file.

openapi-generator version

7.0.0 and 6.6.0, haven't tried other versions.

OpenAPI declaration file content or url

N/A

Generation Details

Steps to reproduce

Use the plugin to generate two yamls that are in a directory.

see https://github.com/Luckl/openapi-gradle-with-root-directory

Related issues/PRs

#15483

Suggest a fix

Change @get:InputFile to @get:InputDirectory