add main to the master branch regex in the default configuration

Author: thetimbecker

docs/input/docs/configuration.md

@@ -283,7 +283,7 @@ upgrade.
 ```yaml
 branches:
   master:
-    regex: ^master
+    regex: ^master|main$
     mode: ContinuousDelivery
     tag: ''
     increment: Patch

src/GitVersionCore.Tests/Configuration/ConfigProviderTests.CanWriteOutEffectiveConfiguration.approved.txt

@@ -31,7 +31,7 @@ branches:
     increment: Patch
     prevent-increment-of-merged-branch-version: true
     track-merge-target: false
-    regex: ^master$
+    regex: ^master|main$
     source-branches:
     - develop
     - release

src/GitVersionCore/Model/Configuration/Config.cs

@@ -115,7 +115,7 @@ public override string ToString()
         public const string HotfixBranchRegex = "^hotfix(es)?[/-]";
         public const string SupportBranchRegex = "^support[/-]";
         public const string DevelopBranchRegex = "^dev(elop)?(ment)?$";
-        public const string MasterBranchRegex = "^master$";
+        public const string MasterBranchRegex = "^master|main$";
         public const string MasterBranchKey = "master";
         public const string ReleaseBranchKey = "release";
         public const string FeatureBranchKey = "feature";