Nuvoton: Update M252 target name (5.15)

[ccli8]

Summary of changes

The target name for Nuvoton's M252 board is resolved to NUMAKER_IOT_M252 from NUMAKER_M252KG (ARMmbed/mbed-os-tools#220). This PR is backport of #13332 to mbed-os 5.15 to also synchronize this change.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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[adbridge]

@ccli8 Please add a proper summary of the changes in this PR rather than just 'backport of ....' . Thank you .

[ccli8]

Please add a proper summary of the changes in this PR rather than just 'backport of ....' . Thank you .

@adbridge Updated the PR summary.

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