[INTERNAL] Update links after code-split / main to v4 branch rename

Author: matz3

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,7 +3,7 @@
 To get it merged faster, kindly review the checklist below:
 
 ## Pull Request Checklist
-- [ ] Reviewed the [Contributing Guidelines](https://github.com/UI5/cli/blob/main/CONTRIBUTING.md#-contributing-code)
-    + Especially the [How to Contribute](https://github.com/UI5/cli/blob/main/CONTRIBUTING.md#how-to-contribute) section 
-- [ ] [No merge commits](https://github.com/UI5/cli/blob/main/docs/Guidelines.md#no-merge-commits)
-- [ ] [Correct commit message style](https://github.com/UI5/cli/blob/main/docs/Guidelines.md#commit-message-style)
+- [ ] Reviewed the [Contributing Guidelines](https://github.com/UI5/cli/blob/v4/CONTRIBUTING.md#-contributing-code)
+    + Especially the [How to Contribute](https://github.com/UI5/cli/blob/v4/CONTRIBUTING.md#how-to-contribute) section 
+- [ ] [No merge commits](https://github.com/UI5/cli/blob/v4/docs/Guidelines.md#no-merge-commits)
+- [ ] [Correct commit message style](https://github.com/UI5/cli/blob/v4/docs/Guidelines.md#commit-message-style)

CONTRIBUTING.md

@@ -1,3 +1,3 @@
 # Contributing to the UI5 CLI
 
-See CONTRIBUTING.md in the [UI5/cli](https://github.com/UI5/cli/blob/main/CONTRIBUTING.md) repository.
+See CONTRIBUTING.md in the [UI5/cli](https://github.com/UI5/cli/blob/v4/CONTRIBUTING.md) repository.

README.md

@@ -24,10 +24,10 @@ UI5 Server documentation can be found here: [ui5.github.io/cli](https://ui5.gith
 The UI5 Server API Reference can be found here: [`@ui5/server`](https://ui5.github.io/cli/v4/api/module-@ui5_server.html)
 
 ## Contributing
-Please check our [Contribution Guidelines](https://github.com/UI5/cli/blob/main/CONTRIBUTING.md).
+Please check our [Contribution Guidelines](https://github.com/UI5/cli/blob/v4/CONTRIBUTING.md).
 
 ## Support
-Please follow our [Contribution Guidelines](https://github.com/UI5/cli/blob/main/CONTRIBUTING.md#report-an-issue) on how to report an issue.
+Please follow our [Contribution Guidelines](https://github.com/UI5/cli/blob/v4/CONTRIBUTING.md#report-an-issue) on how to report an issue.
 
 Please report issues in the main [UI5 CLI](https://github.com/UI5/cli) repository.