Remove support for CodeQL CLI versions older than 2.20.7

[robertbrignull]

Just regular deprecation since we only aim to support the last 4 minor versions of CodeQL.

[Copilot]

Pull Request Overview

This PR removes support for CodeQL CLI version 2.19.4, maintaining support for only the last 4 minor versions as per the project's deprecation policy. The change updates the minimum supported CLI version from 2.19.4 to 2.20.7.

• Updated the oldest supported CLI version constant from 2.19.4 to 2.20.7
• Removed version 2.19.4 from the supported CLI versions configuration
• Added changelog entry documenting the deprecation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
supported_cli_versions.json	Removed v2.19.4 from the list of supported CLI versions
cli.ts	Updated OLDEST_SUPPORTED_CLI_VERSION constant to reflect new minimum version
CHANGELOG.md	Added entry documenting the removal of support for CLI versions older than 2.20.7

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