Improve pull_request_template.md (#22888)

Update `pull_request_template.md` because:

- It's a kind idea to hide the tips. However, it's easier to include
them in the commit message by mistake when you cannot see them. Check
`git log | grep 'Please check the following:'`. So don't hide it, expose
it and help fix it.
- "for backports" is much clearer than "for bug fixes". I saw someone
post a PR to a release branch because they believed it was the right way
for a bugfix.
- "Allow edits by maintainers", or we have to ask the contributor to
update the branch and they could be confused.
- Remind the contributor that the words could be included in the commit
message, to avoid some words like "Hello", "Sorry". If they really need
them, they can separate them with a line, like:

```markdown
Close #xxxx
Because ... Then ... Finally ...
---
Hello, this is my first time opening a pull request. Sorry for any mistakes.
```
And the merger should be careful, check and delete the extra content
before merging.

---------

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

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