[Maven 4] Replace ${maven.multiModuleProjectDirectory} with ${session.rootDirectory}

[gsmet]

This will be when we migrate to Maven 4.
While Maven 3.9.2 also introduces ${session.rootDirectory}, it is not usable in POM files, only in the Maven config itself.

/cc @aloubyansky for your awareness.

Fixes #42430

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[manofthepeace]

This should be removed as well; .mvn/maven.config (for mvn 4)

[gsmet]

Ah yes, it makes perfect sense, let me adjust.

[gsmet]

done!

[quarkus-bot]

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