Remove Gazebo Classic installation branches for Ubuntu 18.04 and 20.04.
The script now only supports Ubuntu 22.04 and 24.04 with Gazebo Harmonic.
Supported Ubuntu LTS versions going forward:
- Ubuntu 24.04 (primary, used in CI and release builds)
- Ubuntu 22.04 (secondary, still supported)
When Ubuntu 26.04 LTS releases we will bump to 26.04/24.04.
Signed-off-by: Ramon Roche <mrpollo@gmail.com>
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