docs: simplify pull request template

[woile]

Description

The current template has a lot of redundancy, the "Type of changes" are already specified on the commits, and the realted issues can be part of the additional context, this reduces the noise when filling the template.

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Steps to Test This Pull Request

1. Open a new pull request
2. Observe

[codecov]

Codecov Report

Merging #243 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #243   +/-   ##
=======================================
  Coverage   96.58%   96.59%           
=======================================
  Files          33       33           
  Lines         908      910    +2     
=======================================
+ Hits          877      879    +2     
  Misses         31       31           

Flag	Coverage Δ
#unittests	96.59% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commitizen/commands/bump.py	90.58% <ø> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/cz/base.py	100.00% <0.00%> (ø)
commitizen/git.py	95.55% <0.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d6e6eb5...ca6bbb2. Read the comment docs.