Remove old feature gates from pre-4.0
Created by: eseliger
Requires a newer Sourcegraph version from https://github.com/sourcegraph/sourcegraph/pull/40446.
This also adds a feature gate for sourcegraph 4.0 to src-cli to warn users when they use a too recent version of src-cli.
Closes https://github.com/sourcegraph/sourcegraph/issues/38969 Closes https://github.com/sourcegraph/sourcegraph/issues/38967
Test plan
Test suite and verified manually the 4.0 banner and previous functionality work.