chore: update examples to use latest release and other small updates

[anitarua]

Addresses the last two tasks of https://github.com/momentohq/dev-eco-issue-tracker/issues/604

• Updates all examples to use the latest release (0.4.0)
• Subscribe examples now use switch expressions to assign subscriptions directly (if using >= Swift 5.9)
• Showed how to use unsubscribe in the topics example and in the docs example
• For the examples that are run in CI, exit(1) whenever an error occurs instead of simply printing errors (examples may build and pass despite warnings/errors #109)
• Runs the cache example in CI instead of just building it
• iOS sample app uses MOMENTO_API_KEY env var instead of hardcoded string
• Updated readme template to use v0.4.0
• Updated links in readmes to point to specifically the api-keys page or cache page as appropriate

[pgautier404]

LGTM!