docs(legacy): clarify that csp hashes could change between minors

[sapphi-red]

Description

#15021 (comment)

Additional context

Should we mention that pinning the version isn't needed if you are getting the value from import { cspHashes } from '@vitejs/plugin-legacy'?

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-cat]

Should we mention that pinning the version isn't needed if you are getting the value from import { cspHashes } from '@vitejs/plugin-legacy'?

This sounds like a good idea to me 👍🏼

[sapphi-red]

Tried to include that pinning is not necessary when using cspHashes. I'm not sure if the text is clear.

[patak-cat]

This reads very well, and it is a good justification as to why we don't consider the changes in the hashes a breaking change