Support specified repositories for plugin DSL

[arssycro]

Is your feature request related to a problem? Please describe.
We have an internal repository behind VPN that this plugin is able to access and update our regular dependencies, but our plugins are not being discovered.

I am guessing this is related to https://github.com/dipien/releases-hub-gradle-plugin/blob/master/releases-hub-gradle-plugin/src/main/java/com/releaseshub/gradle/plugin/artifacts/fetch/ArtifactUpgradeHelper.kt#L49-L52 (introduced for #56).

Describe the solution you'd like
I believe it should use the specified repositories in addition to the Gradle Portal.

Additional context
Here's a trimmed down, made up example of what our settings.gradle.kts looks like:

pluginManagement {
    plugins {
        id("com.internal.plugin") version "2.0.2"
    }

    repositories {
        maven {
            url = uri("https://private.internal.com/repository/maven-releases/")
            mavenContent {
                releasesOnly()
            }
        }
        gradlePluginPortal()
    }
}