Serial: Deprecate the class and promote UnbufferedSerial instead

[hugueskamba]

Summary of changes

Serial provides unbuffered I/O and is using blocking HAL calls. Deprecate the class and promote UnbufferedSerial as it is designed for an interrupt driven application or one that needs to have more control.

Impact of changes

Migration actions required

Documentation

https://github.com/ARMmbed/mbed-os/blob/master/docs/design-documents/drivers/serial/serial.md

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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

@evedon @kjbracey-arm @rajkan01

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[ciarmcom]

@hugueskamba, thank you for your changes.
@kjbracey-arm @rajkan01 @evedon @ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[adbridge]

For #6b8292d236147c82d21fceb77095d032f10ed238 could you please do a rebase and not a merge.

[hugueskamba]

For #6b8292d236147c82d21fceb77095d032f10ed238 could you please do a rebase and not a merge.

There was a GitHub merge conflict which was solved using GitHub.

This fixes the history.

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[hugueskamba]

This force push restores the branch to the sha that was used to run the CI. There was an incorrect force push that changed it. The branch is ready to be merged.