We allow characters in the ID of an ILM policy that will result in an invalid snapshot name.

For example, an ILM policy with the name `mypolicy-*` can be created, however, the generation of the snapshot name in this line:

https://github.com/elastic/elasticsearch/blob/befb6faafc9fca327fd3cea2d0ab328a521c4e65/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/GenerateSnapshotNameStep.java#L81

Results in the `mypolicy-*` being part of the snapshot name, which the snapshot service then disallows. We should strip these invalid characters from the snapshot name.
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/GenerateSnapshotNameStep.java
public GenerateSnapshotNameStep(StepKey key, StepKey nextStepKey, String snapshotRepository) {
        super(key, nextStepKey);
        this.snapshotRepository = snapshotRepository;
    }
    public String getSnapshotRepository() {
        return snapshotRepository;
    }
    @Override
    public ClusterState performAction(Index index, ClusterState clusterState) {
        IndexMetadata indexMetadata = clusterState.metadata().index(index);
        if (indexMetadata == null) {
            
            logger.debug("[{}] lifecycle action for index [{}] executed but index no longer exists", getKey().action(), index.getName());
            return clusterState;
        }
        String policyName = indexMetadata.getLifecyclePolicyName();
        LifecycleExecutionState lifecycleState = indexMetadata.getLifecycleExecutionState();
        
        
        
        if (clusterState.metadata().custom(RepositoriesMetadata.TYPE, RepositoriesMetadata.EMPTY).repository(snapshotRepository) == null) {
            throw new IllegalStateException(
                "repository ["
                    + snapshotRepository
                    + "] is missing. ["
                    + policyName
                    + "] policy for "
                    + "index ["
                    + index.getName()
                    + "] cannot continue until the repository is created or the policy is changed"