build.yml: stop testing on GHC <= 8.6

... the Haskell image of it does not support running node20:

	/__e/node20/bin/node: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.27' not found (required by /__e/node20/bin/node)
	/__e/node20/bin/node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /__e/node20/bin/node)
	/__e/node20/bin/node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by /__e/node20/bin/node)

I could try the following on GitHub actions:

    env:
        ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION: node16
        ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true

as noted here: actions/runner#2906

However, it seems support for this was removed this spring anyway:

	https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

... or will be soon.
So I might as well stop running CI on older GHCs already...

.github/workflows/build.yml

@@ -76,10 +76,6 @@ jobs:
           - '9.0'
           - '8.10'
           - '8.8'
-          - '8.6'
-          - '8.4'
-          - '8.2'
-          - '7.10'
     runs-on: ubuntu-latest
     needs: build-and-test
     container: haskell:${{ matrix.ghc }}