chore: update issue template (googleapis#10695)

- in bug report suggest users provide a go.mod
- update markdown formatting lints
- remove migration issue template. I don't think we ever got issues from this (yay). And things have been migrated for a long time now.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -7,42 +7,52 @@ assignees: ''
 
 ---
 
-**Client**
+## Client
 
 e.g. PubSub
 
-**Environment**
+## Environment
 
 e.g. Alpine Docker on GKE
+e.g. $ go version
 
-**Go Environment**
+## Code and Dependencies
 
-$ go version
-$ go env
-
-**Code**
-
-e.g.
 ```go
 package main
 
 func main() {
-	// ...
+  // ...
 }
 ```
 
-**Expected behavior**
+<details>
+  <summary>go.mod</summary>
+
+```text
+module modname
+
+go 1.23.0
+
+require (
+   // ...
+)
+```
+
+</details>
+
+## Expected behavior
 
 e.g. Messages arrive really fast.
 
-**Actual behavior**
+## Actual behavior
 
 e.g. Messages arrive really slowly.
 
-**Screenshots**
+## Screenshots
 
 e.g. A chart showing how messages are slow. Delete if not necessary.
 
-**Additional context**
+## Additional context
 
 e.g. Started after upgrading to v0.50.0.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -7,15 +7,15 @@ assignees: ''
 
 ---
 
-**Is your feature request related to a problem? Please describe.**
+## Is your feature request related to a problem? Please describe.
 A clear and concise description of what the problem is. Ex. It would be easier to do Y if the libraries
 did X [...]
 
-**Describe the solution you'd like**
+## Describe the solution you'd like
 A clear and concise description of what you want to happen. What should the API be? How would you use it?
 
-**Describe alternatives you've considered**
+## Describe alternatives you've considered
 A clear and concise description of any alternative solutions or features you've considered.
 
-**Additional context**
+## Additional context
 Add any other context or screenshots about the feature request here.