Site Migrations flow: Remove trailing slash from source URL in "Current site address" field on credential collection step

[sixhours]

Remove trailing slash in the URL on this field in the credentials collection step.

[github-actions[bot]]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Migration: The issue pertains directly to the migration process of sites, specifically addressing a URL formatting concern.
• [Feature] Permalinks & Slugs: This label is relevant as the issue discusses modifying the URL format by removing a trailing slash, which relates to permalink settings.
• [Feature] Site Settings: The issue involves a setting that impacts how site addresses are displayed during the credential collection step.