BLE: fix passkey for display being returned reversed

[paul-szczepanek-arm]

Summary of changes

The passkey returned in the ble passkeyDisplay event is reversed. To avoid breaking code for people who figured it out and compensated for it this introduces a deprecation notice. This way anyone who turns off the deprecation notice will also get the passkey returned in correct order.

To summarise:
no action -> no change + deprecation notice
config override -> code returned correctly

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

[] 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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Reviewers

@pan-

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[ciarmcom]

@paul-szczepanek-arm, thank you for your changes.
@pan- @ARMmbed/mbed-os-connectivity @ARMmbed/mbed-os-maintainers please review.

[pan-]

LGTM

[0xc0170]

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[mbed-ci]

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jenkins-ci/mbed-os-ci_build-example-ARM	✔️
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jenkins-ci/mbed-os-ci_cmake-example-test	✔️