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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Fix the field name in a gRPC message. See #2504 for details.

What is the current behavior?

$ arduino-cli core list --json | jq '.platforms[4].releases."1.8.13"'
{
 "name": "Arduino SAMD (32-bits ARM Cortex-M0+) Boards",
 "version": "1.8.13",
 "type": [
 "Arduino"
 ],
 ...

What is the new behavior?

$ arduino-cli core list --json | jq '.platforms[4].releases."1.8.13"'
{
 "name": "Arduino SAMD (32-bits ARM Cortex-M0+) Boards",
 "version": "1.8.13",
 "types": [
 "Arduino"
 ],
 ...

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

Yes

Other information

Fix #2504