Adds a section about `change_column_default`

What does this MR do?

Includes a new section under our migration style guide for the usage of change_column_default. Particularly, specifying that it's not a risky operation for larger tables and can be used in a single transaction

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.

.gitlab/ci/frontend.gitlab-ci.yml

@@ -123,7 +123,7 @@ compile-assets pull-cache:
     - .use-pg9
   dependencies: ["compile-assets", "compile-assets pull-cache", "setup-test-env"]
 
-karma:
+.karma-base:
   extends: .only-code-frontend-job-base
   variables:
     # we override the max_old_space_size to prevent OOM errors
@@ -134,6 +134,9 @@ karma:
     - scripts/gitaly-test-spawn
     - date
     - bundle exec rake karma
+
+karma:
+  extends: .karma-base
   coverage: '/^Statements *: (\d+\.\d+%)/'
   artifacts:
     name: coverage-javascript
@@ -146,7 +149,12 @@ karma:
     reports:
       junit: junit_karma.xml
 
-jest:
+karma-foss:
+  extends:
+    - .karma-base
+    - .only-ee-as-if-foss
+
+.jest-base:
   extends: .only-code-frontend-job-base
   script:
     - scripts/gitaly-test-spawn
@@ -154,6 +162,14 @@ jest:
     - bundle exec rake frontend:fixtures
     - date
     - yarn jest --ci --coverage
+  cache:
+    key: jest
+    paths:
+      - tmp/jest/jest/
+    policy: pull-push
+
+jest:
+  extends: .jest-base
   artifacts:
     name: coverage-frontend
     expire_in: 31d
@@ -164,11 +180,13 @@ jest:
       - tmp/tests/frontend/
     reports:
       junit: junit_jest.xml
+
+jest-foss:
+  extends:
+    - .jest-base
+    - .only-ee-as-if-foss
   cache:
-    key: jest
-    paths:
-      - tmp/jest/jest/
-    policy: pull-push
+    policy: pull
 
 .qa-job-base:
   extends: