test: update changelog validation to match new mdformat

with new mdformat we no longer need to wrap level 2 headings in escaped
brackets, just normal brackets.

(cherry picked from commit 70c295e)
Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

tests/integration_tests/style/test_repo.py

@@ -44,8 +44,8 @@ def test_repo_validate_changelog():
     errors = []
     for lineno, line in enumerate(changelog, start=1):
         if line.startswith("## "):
-            if not re.match(r"^## \\\[.+\\\]$", line):
-                msg = "Level 2 headings (versions) should be wrapped in \\[\\]"
+            if not re.match(r"^## \[.+\]$", line):
+                msg = "Level 2 headings (versions) should be wrapped in []"
                 errors.append((lineno, msg, line))
         if line.startswith("### "):
             if not re.match(r"^### (Added|Changed|Deprecated|Removed|Fixed)$", line):