Update Mbed CoAP to v5.1.8

[ristohuhtala]

Update Mbed CoAP to v5.1.8.

• Fix to blockwise code. Uses block size received in packet, not block size defined in code. (issue: CoAP blockwise code is out of spec and causes hard fault #13761)

Impact of changes

Migration actions required

Documentation

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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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@teetak01 @anttiylitokola

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[ciarmcom]

@ristohuhtala, thank you for your changes.
@teetak01 @anttiylitokola @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started (travis restarted)

[mbed-ci]

Jenkins CI Test : ✔️ SUCCESS

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jobs	Status
jenkins-ci/mbed-os-ci_unittests	✔️
jenkins-ci/mbed-os-ci_build-greentea-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_build-cloud-example-ARM	✔️
jenkins-ci/mbed-os-ci_build-greentea-ARM	✔️
jenkins-ci/mbed-os-ci_dynamic-memory-usage	✔️
jenkins-ci/mbed-os-ci_greentea-test	✔️
jenkins-ci/mbed-os-ci_cloud-client-pytest	✔️