chore: update workbox-build to v4

[haoqunjiang]

Summary

Fixed annoying deprecation warnings of the outdated joi dependencies.
Though it's a major version bump, there seem to be no user-facing breaking changes for vuepress.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
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If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

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• A convincing reason for adding this feature
• Related documents have been updated
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