Rename current_version to installed_version in Update platform

[frenck]

Proposed change

The current_version attribute of the update platform has been renamed to installed_version; to make it more consistent.

Type of change

• Document existing features within Home Assistant
• Document new or changing features which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Removed stale or deprecated documentation

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: Rename current_version to installed_version in Update platform core#69093