Duplicate dependency flexmark-html2md-converter

### Expected Behavior

Maven build without warnings.

### Actual Behaviour

When using Micronaut 4.10.x with the OpenAPI module (through micronaut-platform parent) Maven emits a warning about a duplicate dependency:

```
[WARNING]
[WARNING] Some problems were encountered while building the effective model for foo:bar:jar:0.1
[WARNING] 'dependencyManagement.dependencies.dependency.(groupId:artifactId:type:classifier)' must be unique: com.vladsch.flexmark:flexmark-html2md-converter:jar -> version ${html2md.converter.version} vs ${flexmark.version} @ io.micronaut.platform:micronaut-platform:4.10.7, /.m2/repository/io/micronaut/platform/micronaut-platform/4.10.7/micronaut-platform-4.10.7.pom, line 3131, column 19
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
```

This is probably caused by the the duplicate `flexmark-html2md-converter` entry in https://github.com/micronaut-projects/micronaut-openapi/blob/7.0.x/gradle/libs.versions.toml#L64-L65.

### Steps To Reproduce

Create a new Micronaut 4.10.x project using Maven as the build tool and run `./mvnw clean`

### Environment Information

_No response_

### Example Application

_No response_

### Version

4.10.x

See https://github.com/micronaut-projects/micronaut-platform/issues/2410

Start with gradle/libs.versions.toml around lines 64–65 and compare the dependency entries with the generated micronaut-platform POM. Reproduce the warning by creating a Micronaut 4.10.x Maven project and running ./mvnw clean; done means the duplicate flexmark-html2md-converter warning no longer appears.

Written by the indexing model from the issue text.