Bump fmt to 12.1.0 to fix xcode 26.4 (#56099)

Summary:
Follow-up to
- https://github.com/facebook/react-native/pull/47033
- https://github.com/facebook/react-native/pull/50431

TODO
- [x] bump packages/react-native/third-party-podspecs/fmt.podspec
- [x] bump scripts/releases/ios-prebuild/configuration.js
- [x] packages/react-native/third-party-podspecs/RCT-Folly.podspec
- [x] packages/react-native/gradle/libs.versions.toml
- [x] 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: https://github.com/facebook/react-native/issues/55601

## Changelog:

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

Pull Request resolved: https://github.com/facebook/react-native/pull/56099

Test Plan: RNTester

Reviewed By: alanleedev

Differential Revision: D97358194

Pulled By: cipolleschi

fbshipit-source-id: 3eb578a99a310e3eb77433692bf35502d0d78d24