Bump fmt to 12.1.0 to fix xcode 26.4

[leotm]

Summary:

Follow-up to

• bump Folly to 2024.10.14.00 #47033
• [RN][iOS][0.76] Bump folly to fix xcode 16.3 removed traits #50431

TODO

• bump packages/react-native/third-party-podspecs/fmt.podspec
• bump scripts/releases/ios-prebuild/configuration.js
• packages/react-native/third-party-podspecs/RCT-Folly.podspec
• packages/react-native/gradle/libs.versions.toml
• packages/rn-tester/Podfile.lock
  • rn-rester (main) ios not building on Xcode 26.3, CI currently only supports Xcode 16
  • i cannot downgrade macos26.4b4 locally to macos15 to install Xcode16.4 and bump pods this way
  • so on Xcode 26.3 i've attempted to bump only fmt minimally, ignoring remaining RN 0.86 hashes

Ref: https://github.com/search?q=repo%3Afacebook%2Freact-native+%2211.0.2%22&type=code

however unable to test rn-tester, ios doesn't seem to build on Xcode 26.3 let alone 26.4b
not tested previous versions of Xcode
not tested prior branches/tags to main

rn-tester CI is passing on old macos-15 Xcode 16.4.0 only, ideally this should be extended to stable macos lts and Xcode 26.3 stable first

separate follow-ups to this minimal fmt bump

• bump CI macos-15 xcode 16.1-4 to macos-26 xcode 26.0-3
• bump rn-tester Podfile.lock from RN 0.82 to 0.86 main (aka nightly)
• bump folly to 2025.11.03.00
• bump folly to 2026.03.09.00

Resolve: #55601

Changelog:

[General][Changed] Bump fmt to 12.1.0 to fix Xcode 26.4

Test Plan:

RNTester

[Brett-Best]

Thanks @leotm, now that the RC is out it would be great to get this merged in so testing can commence :)

I think upgrading Folly to latest would be a good idea as well but perhaps it can be done in a separate PR?

[meta-codesync]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this in D97358194.

[meta-codesync]

@cipolleschi merged this pull request in faeef2b.

[react-native-bot]

This pull request was successfully merged by @leotm in faeef2b

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