Pin signet to 0.21.0 and install before fastlane in xcode resource

[Copilot]

Pins signet to 0.21.0 and installs it before fastlane so the pinned version is resolved first, preventing fastlane's dependency resolution from pulling in a newer incompatible version.

Changes

• resources/xcode.rb — added explicit signet 0.21.0 install step ahead of fastlane in the install_gem action

#{gem_bin} install --no-document signet --force --version 0.21.0 && \
#{gem_bin} install --no-document fastlane --force --version 2.229.0 && \
#{gem_bin} install --no-document xcode-install --force