submodule: bump openthread from `8b04e9c` to `a57d927`

[dependabot[bot]]

Bumps openthread from 8b04e9c to a57d927.

Commits

• a57d927 [string] add ParseDigit(), ParseHexDigit(), and other helper functions (#...
• 31ac626 [routing-manager] track whether a discovered router is local device (#10284)
• 0b503a0 [dataset] simplify Clear() methods (#10283)
• f752153 [routing-manager] update and simplify decisions for processing PIO/RIO (#10281)
• See full diff in compare view

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