Increase idle thread stack size for NUCLEO_F091RC target

[deepikabhavnani]

Description

Resolves: #9106

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

CC @geobruce

[ciarmcom]

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

[jeromecoutant]

I don't understand why we need to update this idle-thread-stack-size only for 1 target...

If a user would like to change the value, he can do it within mbed_app.json file

[deepikabhavnani]

@jeromecoutant - PR set to do not merge, this is temporary fix for verification.

@ARMmbed/team-st-mcd - Is it feasible to reduce the idle thread stack size for ST devices? It was achieved earlier but with addition of new commits stack usage is increased.