Update actions/checkout action to v6

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
actions/checkout	action	major	v2 -> v6
actions/checkout	action	major	v3.0.2 -> v6.0.0

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Release Notes

actions/checkout (actions/checkout)

v6

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v5

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v4

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• url-helper.ts now leverages well-known environment variables by @​jww3 in #​1941
• Expand unit test coverage for isGhes by @​jww3 in #​1946

v3

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• Fix: Mark test scripts with Bash'isms to be run via Bash
• Add option to fetch tags even if fetch-depth > 0

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