[installer] Disable definitely-gp by default

[corneliusludmann]

The installer has a config value definitelyGpDisabled that is currently false by default. Since we deprecate definitely-gp soon, we should set it to true by default.

Things to consider:

• Change the default config value in the installer
• Update air-gap.md
• Check if we need to set the value to false for preview envs and SaaS installations