[github actions] rewrite cherry pick template (#144065)

Context: @godofredoc pointed out that the fields in the PR template could be simplified and redesigned. Synced with @itsjustkevin and Kevin come up with [changes in this doc](https://docs.google.com/document/d/1Slppqq32toSwwxUyYTX1t6dGUQpyi0xEWxw4niGWspc/edit?resourcekey=0-v6U_9XVsMFSx0Fvx1rANhg&tab=t.0) which would improve the cherry pick template.

Github only supports a limited number of formatting options. This PR uses some workarounds and makes the following changes:
1. use divider to replace text area since Github markdown doesn't support text area
2. remove \<pre\> tags so texts can wrap around
3. use headings over bold
4. use task lists to identify the starting points of user answers

An example cherry pick template would [look like this](flutter/flutter#143785) after the change.

Author: XilaiZhang

.github/PR_TEMPLATE/PULL_REQUEST_CP_TEMPLATE.md

@@ -1,73 +1,40 @@
-**This pull request is opened against a release branch.<br>
-To request a cherry pick of a commit, please fill in the form below.** (Questions with an asterisk are required.)<br>
+This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/wiki/Flutter-Cherrypick-Process#automatically-creates-a-cherry-pick-request)
+Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.
 
-**To fill in the form, you can edit this PR description and type your answers after the 'My Answer' keywords. <br>
-A flutter domain expert will evaluate this cherry pick request shortly after ALL questions are answered.**
+### Issue Link:
+What is the link to the issue this cherry-pick is addressing?
 
-* Issue Link: What is the link to the issue this cherry-pick is addressing?<br>
+< Replace with issue link here >
 
-<pre>
-  <b>My Answer:</b>
+### Changelog Description:
+Explain this cherry pick in one line that is accessible to most Flutter developers. See [best practices](https://github.com/flutter/flutter/wiki/Hotfix-Documentation-Best-Practices) for examples
 
+< Replace with changelog description here >
 
-</pre>
+### Impact Description:
+What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)
 
-* Changelog Description: Explain this cherry pick in one line that is accessible to most Flutter developers
-        See https://github.com/flutter/flutter/wiki/Hotfix-Documentation-Best-Practices for examples (Bug fix, feature, docs update, ...)<br>
+< Replace with impact description here >
 
-<pre>
-  <b>My Answer:</b>
+### Workaround:
+Is there a workaround for this issue?
 
+< Replace with workaround here >
 
-</pre>
+### Risk:
+What is the risk level of this cherry-pick?
 
-* Impacted Users: Approximately who will hit this issue (ex. all Flutter devs, Windows developers, all end-customers, apps using X framework feature)?<br>
-
-<pre>
-  <b>My Answer:</b>
-
-
-</pre>
-
-* impact_description: What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)<br>
-
-<pre>
-  <b>My Answer:</b>
-
-
-</pre>
-
-* Workaround: Is there a workaround for this issue?<br>
-
-<pre>
-  <b>My Answer:</b>
-
-
-</pre>
-
-* Risk: What is the risk level of this cherry-pick?<br>
-
-<pre>
-  <b>My Answer:</b>
   - [ ] Low
   - [ ] Medium
   - [ ] High
 
-</pre>
+### Test Coverage:
+Are you confident that your fix is well-tested by automated tests?
 
-* Test Coverage: Are you confident that your fix is well-tested by automated tests?<br>
-
-<pre>
-  <b>My Answer:</b>
   - [ ] Yes
   - [ ] No
 
-</pre>
-
-* Validation Steps: What are the steps to validate that this fix works?<br>
-
-<pre>
-  <b>My Answer:</b>
-
+### Validation Steps:
+What are the steps to validate that this fix works?
 
-</pre>
+< Replace with validation steps here >