Hello world example: Inform deprecation before user encounters it

[acastellane]

URL to the relevant documentation

https://docs.quantum.ibm.com/guides/hello-world

Select all that apply

• typo
• code bug
• out-of-date content
• broken link
• other

Describe the fix.

Note: This video uses the QiskitRuntimeService.get_backend method, which has since been deprecated. Use QiskitRuntimeService.backend instead.

Suggestion to move this note before the video, so that user is informed before using a deprecated command.

[javabster]

Thanks for opening this issue and giving your feedback, this is a good suggestion and we will implement it when we can. You are also welcome to open a PR with this fix too if you like