Update the maven sync worker

🎈 Context

The maven package registry uses xml files (named maven-metadata.xml) to describe and list the available versions of a given package within the packages registry. A more in depth analysis is available here.

In !55207 (merged), we added a background worker that syncs the versions between the ones existing in the database and the ones that are listed in the most recent maven-metadata.xml file. This way, the xml file can't reference non existing versions.

This xml file is not hosted in the usual Packages::Package record: it is in a record that doesn't have a version set, also called the versionless package. This is expected because the maven-metadata.xml is not bound to a given version but only to a given package name.

!55207 (merged) was deployed on gitlab.com and we discovered that not all the maven artifacts have a versionless package.

This is currently expected by the worker and so, if the versionless package is not present, a sync error is risen. See #324288 (closed).

🔍 What does this MR do?

• Relax the maven sync worker so that we not finding the versionless package, the worker ends its execution normally
• Updates the related specs

Screenshots (strongly suggested)

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Does this MR meet the acceptance criteria?

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