MAINT: Rename master to main

RELEASE_GUIDELINES.md

@@ -21,13 +21,13 @@ guidelines of the CaosDB Project
 3. Update the version property in [pom.xml](./pom.xml) (probably this means to
    remove the `-SNAPSHOT`.
 
-4. Merge the release branch into the master branch.
+4. Merge the release branch into the main branch.
 
-5. Tag the latest commit of the master branch with `v<VERSION>`.
+5. Tag the latest commit of the main branch with `v<VERSION>`.
 
 6. Delete the release branch.
 
-7. Merge the master branch back into the dev branch.
+7. Merge the main branch back into the dev branch.
 
 8. Update the version property in [pom.xml](./pom.xml) for the next
    developlement round (with a `-SNAPSHOT` suffix).