Update markdown files for `main` branch

Coding standards and PR template now reference `main`.

.github/pull_request_template.md

@@ -37,10 +37,9 @@ If the changes are not amenable to automated testing, please explain why not and
 
 <!--
 Tick one of the following boxes [x] to help us understand if the base branch for the PR is correct.
-see [CODING_STANDARDS.md](https://github.com/MariaDB/server/blob/-/CODING_STANDARDS.md) for the latest versions.
 -->
 ## Basing the PR against the correct MariaDB version
-- [ ] *This is a new feature or a refactoring, and the PR is based against the latest MariaDB development branch.*
+- [ ] *This is a new feature or a refactoring, and the PR is based against the `main` branch.*
 - [ ] *This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.*
 
 <!--

CODING_STANDARDS.md

@@ -48,7 +48,7 @@ git push --force my_branch
 ### Target branch
 
 Pull requests should be based against the correct MariaDB version.
-New features should be based against the latest MariaDB development branch, which is the current GitHub default branch: https://github.com/MariaDB/server/blob/-/VERSION
+New features should be based against the `main` branch, which is the GitHub default branch.
 Bug fixes should be based against the earliest maintained branch in which the bug can be reproduced.
 The earliest maintained branch is found at https://mariadb.org/about/#maintenance-policy.