Disable mbed 2 and unsupported targets

[MarceloSalazar]

Summary of changes

Mbed 2 targets are not expected to be supported on master and for the Mbed OS 6 release.
The following changes are introduced:

• Remove "2" from release_versions as there is no Mbed 2 and these targets won't work as such
• Remove LPC11U68 and KL27Z from targets.json as these are Mbed 2 only. They require migration to Mbed OS 6 Baremetal and can be introduced in the future.
• Remove MAX32620HSP as there is no release_versions and is considered unsupported - see Maxim targets are not currently enabled #11233

Impact of changes

These targets continue to be available in the 5.15 branch.
Mbed OS 6 won't support these targets unless are re-introduced.

Migration actions required

none

Documentation

None

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[X] 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

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[ciarmcom]

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

[0Grit]

Been waiting for this one for years. Great work everybody!

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts

[jeromecoutant]

* Remove "2" from release_versions as there is no Mbed 2 and these targets won't work as such

Hi
Why don't you remove all release_versions. It looks OK for STM32 targets...?

[0xc0170]

@jeromecoutant That will come but tools need some fixing as well to get this in (another PR is fixing part of the problem)

[MarceloSalazar]

FYI @ARMmbed/team-nxp