Modernize this code base

[lorentey]

• Bump required toolchain version to Swift 6.1.
• Bump minimum deployment target to macOS 15, iOS 18, watchOS 11, tvOS 18, visionOS 2.
• Fix some glaring issues with concurrency safety; add Sendable where required.
• Adopt the use of SuspendingClock and Duration from the Swift Standard Library for measuring elapsed time.
• Use struct Atomic from the stdlib to do basic synchronization.
• Update dependencies to their latest versions.
• Stop supporting a built-in charts library -- the library json now always needs to be explicitly specified on the command line.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering my changes (if appropriate).
• I've verified that my change compiles and works correctly, and does not break any existing tests.
• I've updated the documentation (if appropriate).

[lorentey]

@swift-ci test

[lorentey]

Run which yamllint || (apt -q update && apt install -yq yamllint)
/opt/pipx_bin/yamllint
Downloading default yamllint config file
./.spi.yml
  Error: 4:52 [new-line-at-end-of-file] no new line character at the end of file

😠

[lorentey]

  Error: 5:1 [empty-lines] too many blank lines (1 > 0)

Oh come on! Cc @shahmishal

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test