Update to latest plugin

[jdpgrailsdev]

What

• Update to latest Open API generator Gradle plugin

How

• Bump to version 6.2.1 to avoid this issue with snapshot resolution:

FAILURE: Build failed with an exception.

* Where:
Build file '/actions-runner/_work/airbyte/airbyte/airbyte-api/build.gradle' line: 6

* What went wrong:
Plugin [id: 'org.openapi.generator', version: '6.1.0-SNAPSHOT', artifact: 'org.openapitools:openapi-generator-gradle-plugin:6.1.0-SNAPSHOT'] was not found in any of the following sources:

- Gradle Core Plugins (plugin is not in 'org.gradle' namespace)
- Plugin Repositories (could not resolve plugin artifact 'org.openapitools:openapi-generator-gradle-plugin:6.1.0-SNAPSHOT')

  Searched in the following repositories:
    Gradle Central Plugin Repository
    maven(https://oss.sonatype.org/content/repositories/snapshots)

Deprecated Gradle features were used in this build, making it incompatible with Gradle 8.0.

Recommended reading order

1. build.gradle

[supertopher]

Seems sane to topher