Update haskell-actions/run-ormolu action to v21

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
haskell-actions/run-ormolu	action	major	v14 → v21

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Release Notes

haskell-actions/run-ormolu (haskell-actions/run-ormolu)

v21

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• Rewrote the action in TypeScript (src/index.ts), bundled into
  dist/index.js with ncc. No change in behavior or inputs.
• Added a reproducible build with Nix (flake.nix): nix build produces the
  bundled dist/ hermetically, and nix develop provides a pinned Node.js
  dev shell. CI now verifies that the committed dist/ matches a fresh build.
• Updated the @actions/* dependencies to their current major versions. As
  a side effect, @actions/tool-cache 2.x pulls in undici, which
  increases the size of the bundled dist/index.js. This does not affect
  the action's runtime performance.

v20

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• Switch to the new official upstream of the project and default to the
  version 0.8.2.0.

v19

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• Specifies that it wants to run on Node.js 24 explicitly, so you won't be
  getting any warnings about Node.js 20 anymore.

v18

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• Use Ormolu 0.8.1.1 as the latest version.

v17

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• Uses Ormolu 0.8.0.0 as the latest version.

v16

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• Uses Ormolu 0.7.7.0 as the latest version. On MacOS runners aarch64
  version of Ormolu will be used.

v15

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• Uses Ormolu 0.7.4.0 as the latest version.

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