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• 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)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fixes a command and related gRPC function returning incorrect data.

• What is the current behavior?

The core list command doesn't return the correct latest available version for outdated cores.

$ arduino-cli core list
ID           Installed Latest Name                                        
arduino:samd 1.8.10    1.8.10 Arduino SAMD Boards (32-bits ARM Cortex-M0+)
$ arduino-cli core list --updatable
ID           Installed Latest Name                                        
arduino:samd 1.8.10    1.8.10 Arduino SAMD Boards (32-bits ARM Cortex-M0+)

Same thing for the gRPC interface function.

• What is the new behavior?

core list now correctly the latest available version of the listed cores.

$ arduino-cli core list
ID           Installed Latest Name                                        
arduino:samd 1.8.10    1.8.11 Arduino SAMD Boards (32-bits ARM Cortex-M0+)
$ arduino-cli core list --updatable
ID           Installed Latest Name                                        
arduino:samd 1.8.10    1.8.11 Arduino SAMD Boards (32-bits ARM Cortex-M0+)

Same thing for gRPC interface function.

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Partly fixes #1401.

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