* Remove unused variables

* Introduce a version catalog

This commit replaces the "dependencyVersion" and "dependencyModuleVersion"
calls with Gradle's version catalogs. In the process, I tried to clarify
what belongs to the BOM (as "managed" dependencies") vs what is not.

Currently this breaks BOM verification module, this will be fixed in
another commit. It will also probably break the automatic upgrading of
dependencies. However, the result is much cleaner: there are no duplicate
coordinates anymore in build files.

* Publish a version catalog alongside the BOM

This commit introduces a new artifact published alongside the BOM:
a Gradle _version catalog_. A version catalog is not a replacement
for a BOM. It can be seen as an alternative, or a complement to a
BOM file.

For example, a user can import the Micronaut version catalog in
their settings file by doing:

```gradle
dependencyResolutionManagement {
    versionCatalogs {
        create("mn") {
            from("io.micronaut:micronaut-bom:3.0.1-SNAPSHOT")
        }
    }
}
```

Gradle will then generate _version accessors_, which allows
replacing dependency notations from:

```gradle
implementation "ch.qos.logback:logback-classic"
```

to:

```gradle
implementation mn.logback
```

Those accessors are _type safe_. Version catalogs should
really be seen as "recommendations". Without applying a
BOM in addition, the versions declared in a catalog have
_no impact_ on dependency resolution. Therefore, it is
often recommended to apply both the BOM _and_ use catalogs
to declare dependencies.

It comes with an advantage, which is that versions declared
in the BOM can be overridden by the user:

```gradle
dependencyResolutionManagement {
    versionCatalogs {
        create("mn") {
            from("io.micronaut:micronaut-bom:3.0.1-SNAPSHOT")
            version("snakeyaml") {
                strictly("1.28")
            }
        }
    }
}
```

* Workaround JSON ordering issue under JDK 15 on CI

* Integrate Jackson to the BOM

It looks like it was a mistake that it wasn't present in the BOM already.