Resolves #776: onlyUpgradable change the filter to versions where the current version is not the latest one

[jarmoniuk]

Root cause of the issue: VersionsHelper.lookupDependenciesUpdates actually returns all dependency versions, and not just updates.

The problem manifested itself when the currently used version was the latest, but there were more versions reported (older than the current one).

This is now corrected, i.e. the method returns only updates, in contrast to lookupArtifactVersions.

onlyUpgradableOnly looks to work fine now. Updated the unit tests to test also for cases when the version used by an artifact is the latest version, but there are more available versions (older than the current one).

Removed some redundant classes implementing Callable, which could very easily be inlined which results in a much cleaner code.

Also, as a minor pedantic fix, corrected the "splitted" [sic] to "split" in one method name.

[jarmoniuk]

@slawekjaranowski Please review and merge.

[jarmoniuk]

Converting to draft -- I'll quickly try to do it properly.

Looks like it's now working and is done properly.