Adds acceptance criteria to the issue templates (#361)

# Description

Adds the "Acceptance Criteria" section to existing issue templates.
Also, the MR cleans up any unnecessary fields to avoid bloating in the
templates.

Fixes #359 

## Type of change

- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have run all the tests with `./orbit.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

.github/ISSUE_TEMPLATE/bug.md

@@ -45,3 +45,10 @@ Add any other context about the problem here.
 
 - [ ] I have checked that there is no similar issue in the repo (**required**)
 - [ ] I have checked that the issue is not in running Isaac Sim itself and is related to the repo
+
+### Acceptance Criteria
+
+Add the criteria for which this task is considered **done**. If not known at issue creation time, you can add this once the issue is assigned.
+
+- [ ] Criteria 1
+- [ ] Criteria 2

.github/ISSUE_TEMPLATE/proposal.md

 ---
 name: Proposal
-about: Propose changes that are not fixes bugs
+about: Propose changes that are not bug fixes
 title: "[Proposal] Proposal title"
 ---
 
@@ -17,10 +17,6 @@ Is your feature request related to a problem? e.g.,"I'm always frustrated when [
 
 If this is related to another GitHub issue, please link here too.
 
-### Pitch
-
-A clear and concise description of what you want to happen.
-
 ### Alternatives
 
 A clear and concise description of any alternative solutions or features you've considered, if any.
@@ -32,3 +28,10 @@ Add any other context or screenshots about the feature request here.
 ### Checklist
 
 - [ ] I have checked that there is no similar issue in the repo (**required**)
+
+### Acceptance Criteria
+
+Add the criteria for which this task is considered **done**. If not known at issue creation time, you can add this once the issue is assigned.
+
+- [ ] Criteria 1
+- [ ] Criteria 2