Bump actions/checkout from 4 to 6 #93

Job	Run time
test-default-parameters (ubuntu-latest)	20s
test-default-parameters (windows-latest)	1m 0s
test-custom-pre-build-command-bash (macos-latest)	32s
test-custom-pre-build-command-bash (ubuntu-latest)	22s
test-custom-pre-build-command-bash (windows-latest)	1m 7s
test-custom-mkdocs-version (ubuntu-latest)	22s
test-default-parameters (macos-latest)	26s
test-custom-mkdocs-version (windows-latest)	1m 15s
test-custom-pre-build-command-pwsh (macos-latest)	29s
test-custom-output-directory (macos-latest)	28s
test-custom-mkdocs-version (macos-latest)	27s
test-custom-pre-build-command-pwsh (windows-latest)	1m 15s
test-custom-output-directory (ubuntu-latest)	19s
test-custom-pre-build-command-pwsh (ubuntu-latest)	28s
test-publish-to-pages	33s
test-custom-output-directory (windows-latest)	1m 4s
test-custom-config-file (ubuntu-latest)	21s
test-custom-requirements-path (ubuntu-latest)	20s
test-custom-requirements-path (windows-latest)	1m 9s
test-custom-config-file (windows-latest)	2m 2s
test-custom-requirements-path (macos-latest)	34s
test-custom-config-file (macos-latest)	30s
	15m 23s

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