fix: Update legacy 'error' CSS class to 'notice notice-error'

[debabratakarfa]

Remove <strong> tags from admin error notices by adding new CSS class notice notice-error

Description

This PR addresses the inconsistent use of bold formatting in admin error notices. Currently, many error messages are wrapped entirely in <strong> tags, making the whole message bold. This PR aims to remove these unnecessary <strong> tags, improving readability and consistency with other admin notices.

Changes proposed in this Pull Request:

• Remove <strong> tags that wrap entire error messages
• Retain bold formatting only for specific terms that require emphasis within messages
• Update the legacy CSS class error to the more modern notice notice-error

Trac ticket: https://core.trac.wordpress.org/ticket/50402

Props dkarfa, audrasjb, joedolson.

[github-actions]

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[github-actions]

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 59960
GitHub commit: d5a3a14

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.