pyln-proto: use coincurve 21 for python <=3.13 and add self-compiled wheels for python 3.14

[daywalker90]

coincurve 20 is broken for all python versions since it now pulls in a transient dependency that turned a warning into an error:

× Failed to build `coincurve==20.0.0`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `hatchling.build.build_wheel` failed (exit status: 7)

      [stderr]
      ERROR: Use build.verbose instead of cmake.verbose for scikit-build-core >= 0.10

      hint: This usually indicates a problem with the package or the build environment.
  help: `coincurve` (v20.0.0) was included because `cln-meta-project` (v0.1.0) depends on `pyln-proto` (v26.6.2) which depends on `coincurve`

coincurve 21 does not have this issue but it does have another issue with python 3.14:

Resolved 1 package in 289ms
  × Failed to build `coincurve==21.0.0`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `hatchling.build.build_wheel` failed (exit status: 1)

      [stderr]
      Traceback (most recent call last):
        File "<string>", line 11, in <module>
          wheel_filename = backend.build_wheel("/home/user/.cache/uv/builds-v0/.tmpfcwWrQ", {}, None)
        File "/home/user/.cache/uv/builds-v0/.tmpNmyA8G/lib/python3.14/site-packages/hatchling/build.py", line 58, in build_wheel
          return os.path.basename(next(builder.build(directory=wheel_directory, versions=["standard"])))
                                  ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/home/user/.cache/uv/builds-v0/.tmpNmyA8G/lib/python3.14/site-packages/hatchling/builders/plugin/interface.py", line 149, in build
          build_hook.initialize(version, build_data)
          ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
        File "/home/user/.cache/uv/sdists-v9/pypi/coincurve/21.0.0/sA4L50tQMHeL37XJdHnME/src/hatch_build.py", line 34, in initialize
          raise RuntimeError(message)
      RuntimeError: Expected exactly one LICENSE file in cffi distribution, got 0

      hint: This usually indicates a problem with the package or the build environment.

coincurve for python 3.14 is fixed upstream but there is no release yet, so we use self-compiled wheels just for python 3.14

Changelog-None

Important

26.09 FREEZE August 5th: Non-bugfix PRs not ready by this date will wait for 26.12.

RC1 is scheduled on August 17th

The final release is scheduled for September 7th.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade