submodule: bump openthread from `ff3cc7e` to `148fa47`

[dependabot[bot]]

Bumps openthread from ff3cc7e to 148fa47.

Commits

• 148fa47 [thread-cert] use unittest assert methods (#9936)
• 9d6321b [lib] include (void) in function prototype in reset_util.h (#9934)
• c89d4f0 [routing-manager] update use of Ip6::Nd::Option enumerator (#9931)
• See full diff in compare view

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