Replace Renovate with Dependabot

[simonbasle]

This PR replaces Renovate with Dependabot.

Despite being less configurable, Dependabot can target a non-default branch.

One limitation is that Dependabot currently doesn't process version catalogs, even in the standard format of gradle/libs.versions.toml.
It should however be able to process version strings imported from a .gradle file as long as the filename contains dependencies` (see source).
So unfortunately this means rolling back version catalogs, at lest partially.

The version declarations broadly go into 4 categories:

1. Reactor versions that we always edit during the release
2. Versions that we want to more closely monitor, or freeze
3. Plugins
4. Other libraries

While (4) can and should be managed by Dependabot, the first three categories cannot or shouldn't.
In this PR I chose to keep (1/2/3) in the libs.versions.toml version catalog and split out versions from (4) into a new gradle/dependencies.gradle file to be imported.

That file would thus be understood, monitored and updated by Dependabot only, while we continue to make manual edits to gradle/libs.versions.toml (and root gradle.properties).

Drawbacks

The plugins won't be auto-updated anymore and will need manual monitoring and updates.

Alternatives

This is more a question of style but in the current state, we reintroduce the artifact coordinates in each build.gradle (eg. implementation "group:my-artifact:$myArtifactVersion").
I've found one blog post that uses a different style where the whole coordinates are defined inside the dependencies.gradle file in a dictionnary:

// in `gradle/dependecies.gradle`:
libs = { 
    myArtifact : "group:my-artifact:$myArtifactVersion"
}

// in individual `build.gradle` files
dependencies {
    implementation libs.myArtifact
}

See blog post.
This could be considered as a further improvement.

[kkocel]

One point to drawbacks - dependabot does not support Gradle wrapper updates

[chemicL]

Superseded by #3377.