Bump external/fmt from `a8a73da` to `492a99c`

[dependabot[bot]]

Bumps external/fmt from a8a73da to 492a99c.

Commits

• 492a99c Fix error: 'char_traits<custom_char>' is deprecated: char_traits<T> for T not...
• 3baaa8d Update docs
• 0e01e46 Implement nested formatter
• f6ca4ea Avoid a space in the UDL definition (#3610)
• See full diff in compare view

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[Idhrendur]

@dependabot squash and merge