Remove milestone repository

Spring uses central for milestones now

Author: rwinch

.github/dependabot.yml

@@ -1,8 +1,5 @@
 version: 2
 registries:
-  spring-milestones:
-    type: maven-repository
-    url: https://repo.spring.io/milestone
   shibboleth:
     type: maven-repository
     url: https://build.shibboleth.net/maven/releases
@@ -17,7 +14,6 @@ updates:
     labels:
       - 'type: dependency-upgrade'
     registries:
-      - spring-milestones
       - shibboleth
     ignore:
       - dependency-name: com.nimbusds:nimbus-jose-jwt
@@ -44,7 +40,6 @@ updates:
     labels:
       - 'type: dependency-upgrade'
     registries:
-      - spring-milestones
       - shibboleth
     ignore:
       - dependency-name: com.nimbusds:nimbus-jose-jwt
@@ -72,7 +67,6 @@ updates:
     labels:
       - 'type: dependency-upgrade'
     registries:
-      - spring-milestones
       - shibboleth
     ignore:
       - dependency-name: com.nimbusds:nimbus-jose-jwt
@@ -104,7 +98,6 @@ updates:
     labels:
       - 'type: dependency-upgrade'
     registries:
-      - spring-milestones
       - shibboleth
     ignore:
       - dependency-name: com.nimbusds:nimbus-jose-jwt