Update Mbed CoAP to v5.1.7

[teetak01]

Summary of changes

Update Mbed CoAP to v5.1.7.

• Removed comparison of IP addresses when validating message resending. This avoids unnessary network errors due to load balancers causing frequent IP address changes.

Impact of changes

Migration actions required

Documentation

None

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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

[x] 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

@yogpan01 @anttiylitokola

[yogpan01]

@0xc0170 Upstreaming CoAP error fixes to MASTER. Please take these in so they will make to next OS release patch.

[ciarmcom]

@teetak01, thank you for your changes.
@yogpan01 @anttiylitokola @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

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jenkins-ci/mbed-os-ci_greentea-test	✔️
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