submodule: bump openthread from `f12dfaf` to `9f6921c`

[dependabot[bot]]

Bumps openthread from f12dfaf to 9f6921c.

Commits

• 9f6921c [core] optimize #include directives in core cpp files (#10717)
• c6a4657 [diag] add flag to enable CSMA/CA (#10723)
• fa3509e [tcplp] fix an issue that occurs when using LTO and TCP is not enabled (#10726)
• ba3bce2 [otci] update package info (#10706)
• f07bcc2 [mle] rename Mle::Handle{Command}() methods to indicate constraints (#10721)
• 62df7e9 [message] clarify partial read behavior of Read() vs ReadByte() (#10719)
• 5070adb [settings] simplify saving of Border Agent ID (#10701)
• See full diff in compare view

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[jwhui]

@dependabot recreate

[dependabot[bot]]

Superseded by #803.