gitlab: Reduce size of template headings

.gitlab/issue_templates/bug.md

-# Summary
+## Summary
 
 Write a brief description of the issue.
 
-# Steps to reproduce
+## Steps to reproduce
 
 Please provide a set of concrete steps to reproduce the issue.
 
-# Expected behavior
+## Expected behavior
 
 What do you expect the reproducer described above to do?
 
-# Environment
+## Environment
 
 * GHC version used: 
 

.gitlab/issue_templates/feature_request.md

-# Motivation
+## Motivation
 
 Briefly describe the problem your proposal solves and why this problem should
 be solved.
 
-# Proposal
+## Proposal
 
 Describe your proposed feature here.