Docs: greeting extension fixes

[aloubyansky]

The content is a bit out of date now.

• we do not require an existing pom.xml to run create-extension;
• I couldn't get the proposed test to pass (although I could build the extension skipping the tests and use it in an app);
• I am not sure what the debugging chapter is suggesting, tbh. We also do not support dev mode for extension projects;
• quarkus-platform-bom does not exist yet.

[gsmet]

Used the variable.

[gsmet]

I changed it back to 1.8+ as we are still supporting 1.8 and all the other guides reference 1.8. Better update them all at once (and make it a variable in passing).