Use Che Code in test cases instead of Theia

[AObuchow]

Now that Theia will no longer be officially supported, the test data we use should aim to use Che Code instead of Theia.

@nickboldt has identified the following test data as needing to be updated:

• pkg/library/flatten/testdata/container-contributions/
• pkg/library/flatten/testdata/implicit-container-contributions/
• pkg/library/flatten/testdata/implicit-spec-contributions/
• pkg/library/flatten/testdata/k8s-ref/
• pkg/library/flatten/testdata/spec-contributions/
• pkg/library/lifecycle/testdata/postStart/

[AObuchow]

After some thought, I don't think it's necessary to remove the test data listed in the issue description: the tests are still valid even if Theia is no longer officially supported.

However, we should probably remove the theia-latest sample, as the URL it's using for the theia editor devfile is down, and I can't seem to find a replacement URL.

[nickboldt]

given use of theia was...

• deprecated in DS 3.5 (Che 7.60), and
• removed from DS 3.6 (Che 7.64), and that
• the current release is 7.70...

I would argue that having any mention / use of theia in tests/code is no longer recommended. Updating the tests to focus on vscode would be better than keeping them and having them still mention theia, which is no longer a supported editor in Che.