Change update --outdated flag to --show-outdated

[silvanocerza]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?
  Small feature change.

• What is the current behavior?
  To show outdated cores and libraries after running update you must use the flag --outdated.

• What is the new behavior?
  To show outdated cores and libraries after running update you must use the flag --show-outdated.

• Does this PR introduce a breaking change?
  Not really.

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[umbynos]

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