Docs: Replace @since x.y.z with actual version in WP_Roles and capabilities docs.

[solankisoftware89]

This PR replaces placeholder @SInCE x.y.z tags with the accurate version number 6.9.0 in:

• wp-includes/class-wp-roles.php
• wp-includes/capabilities.php

These updates follow WordPress documentation standards and align with Core ticket #64366.

Trac ticket: https://core.trac.wordpress.org/ticket/64366

Fixes #64366.

[github-actions]

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Props solankisoftware.

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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: 61354
GitHub commit: 7800b8e

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