[optimize]Refactor the dubbo-demo module

[heliang666s]

What is the purpose of the change?

1. Remove dubbo-demo-xml, dubbo-demo-annotation, dubbo-demo-generic-call and dubbo-demo-interface.
2. Change all default protocols from dubbo to triple.
3. Update README.md.

Checklist

• [x] Make sure there is a GitHub_issue field for the change.
• [x] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• [x] Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• [x] Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 2.19%. Comparing base (ef6e8bd) to head (0261a18). Report is 55 commits behind head on 3.3.

:exclamation: Current head 0261a18 differs from pull request most recent head 2af91b4

Please upload reports for the commit 2af91b4 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 3.3 #14871 +/- ## ============================================= - Coverage 37.53% 2.19% -35.34% ============================================= Files 1813 26083 +24270 Lines 80737 1389674 +1308937 Branches 12005 220210 +208205 ============================================= + Hits 30304 30566 +262 - Misses 46042 1354670 +1308628 - Partials 4391 4438 +47 ```

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[AlbumenJ]