delete previous version static caches from CacheStorage

cache-delete.js

/**
suppose, static caches are categorized as static-imgs-v1, static-js-v1...
*/
const version = 'v2';
const staticPrefix = 'static-'
caches.keys().then(function (cacheNames) {
  return Promise.all(
    cacheNames.filter(function (cacheName) {
      return cacheName.startsWith(staticPrefix) &&
        !cacheName.endsWith(version);
    }).map(function (cacheName) {
      return caches.delete(cacheName);
    })
  );
})