.github: rename "master" branch references to "main"

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

.github/ISSUE_TEMPLATE/distributors-application.md

@@ -20,9 +20,9 @@ meeting the membership criteria.
 **4. Be a participant and active contributor in the community.**
 
 **5. Accept the Embargo Policy.**
-<!-- https://github.com/etcd-io/etcd/blob/master/security/security-release-process.md#disclosures -->
+<!-- https://github.com/etcd-io/etcd/blob/main/security/security-release-process.md#disclosures -->
 
 **6. Be willing to contribute back.**
-<!-- Per https://github.com/etcd-io/etcd/blob/master/security/security-release-process.md#patch-release-and-public-communication -->
+<!-- Per https://github.com/etcd-io/etcd/blob/main/security/security-release-process.md#patch-release-and-public-communication -->
 
 **7. Have someone already on the list vouch for the person requesting membership on behalf of your distribution.**

.github/SECURITY.md

@@ -1,2 +1,2 @@
 
-Please read https://github.com/etcd-io/etcd/blob/master/security/README.md.
+Please read https://github.com/etcd-io/etcd/blob/main/security/README.md.

.github/workflows/codeql-analysis.yml

@@ -13,10 +13,10 @@ name: "CodeQL"
 
 on:
   push:
-    branches: [ master, release-0.4, release-2.0, release-2.1, release-2.2, release-2.3, release-3.0, release-3.1 ]
+    branches: [ main, release-0.4, release-2.0, release-2.1, release-2.2, release-2.3, release-3.0, release-3.1 ]
   pull_request:
     # The branches below must be a subset of the branches above
-    branches: [ master ]
+    branches: [ main ]
   schedule:
     - cron: '20 14 * * 5'
 

.github/workflows/self-hosted-linux-arm64-graviton2-tests.yml

@@ -2,9 +2,9 @@ name: Linux ARM64 Graviton2
 
 on:
   push:
-    branches: [master]
+    branches: [main]
   pull_request:
-    branches: [master]
+    branches: [main]
 
 jobs:
   test: