Bump old hls version and upgrade test runner to macos-latest #960
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Following https://haskell-language-server.readthedocs.io/en/latest/support/ghc-version-support.html, HLS has dropped ghc-8.10.7 after hls-2.2.0.0, so we can replace 8.10.4 with 8.10.7 and hope it can make our test more stable...