[CI] -- Update to use official Swift 5.3 release images

Author: Mordil

.gitlab/ci/amazonlinux-2.yml

@@ -7,7 +7,7 @@ swift trunk:
 
 swift 5.3:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.3-amazonlinux2
+  image: swift:5.3-amazonlinux2
 
 swift 5.2:
   extends: .unit-test

.gitlab/ci/centos-7.yml

@@ -15,7 +15,7 @@ swift trunk:
 
 swift 5.3:
   extends: .centos7-test-workaround
-  image: swiftlang/swift:nightly-5.3-centos7
+  image: swift:5.3-centos7
 
 swift 5.2:
   extends: .centos7-test-workaround

.gitlab/ci/centos-8.yml

@@ -7,7 +7,7 @@ swift trunk:
 
 swift 5.3:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.3-centos8
+  image: swift:5.3-centos8
 
 swift 5.2:
   extends: .unit-test

.gitlab/ci/ubuntu-bionic.yml

@@ -7,7 +7,7 @@ swift trunk:
 
 swift 5.3:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.3-bionic
+  image: swift:5.3-bionic
 
 swift 5.2:
   extends: .unit-test

.gitlab/ci/ubuntu-focal.yml

@@ -7,7 +7,7 @@ swift trunk:
 
 swift 5.3:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.3-focal
+  image: swift:5.3-focal
 
 swift 5.2:
   extends: .unit-test

.gitlab/ci/ubuntu-xenial.yml

@@ -7,7 +7,7 @@ swift trunk:
 
 swift 5.3:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.3-xenial
+  image: swift:5.3-xenial
 
 swift 5.2:
   extends: .unit-test