fix(settings): Update delete passkey modal body text

[vpomerleau]

Because

• Passkey modal body text did not match latest design

This pull request

• Update the body text and ftl string

Issue that this pull request solves

Closes: FXA-13969

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• My commit is GPG signed.
• [ \x] If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
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[Copilot]

Pull request overview

Updates the passkey deletion confirmation modal copy in FxA Settings to align with the latest design language, including corresponding Fluent (FTL) strings and a test expectation update.

Changes:

• Updated the delete-passkey modal body text in PasskeyDeleteModal.
• Renamed/updated the Fluent string id and its English value.
• Updated the unit test to assert the new modal body copy.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/fxa-settings/src/components/Settings/SubRow/index.tsx	Updates the delete passkey modal’s localized body text (and references the new FTL id).
packages/fxa-settings/src/components/Settings/SubRow/index.test.tsx	Updates assertions to match the new modal body copy.
packages/fxa-settings/src/components/Settings/SubRow/en.ftl	Updates the English Fluent string for the modal body (new -v2 id).

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