我能够在October 6th上构建和发布我的flutter应用程序，今天我在Octuber 30th上返回，我得到了这个构建错误：

• Could not resolve io.grpc:grpc-core:[1.28.0].
• Could not HEAD 'https://jcenter.bintray.com/io/grpc/grpc-core/maven-metadata.xml'.
• Read timed out

cloud_firestore似乎有问题，不知何故，它仍然试图从jcenter读取，我被告知它关闭了，不支持新的仓库，但问题是，我没有改变一行，也没有更新任何包，因为我上次发布和构建，是什么导致了这个问题？
我记得几个月前我也遇到过类似的问题，但在我用mavenCentral替换jcenter后，一切都得到了修复，但这次它又回来了。

Launching lib\main.dart on Android SDK built for x86 in debug mode...

FAILURE: Build failed with an exception.

* What went wrong:
Could not determine the dependencies of task ':app:processDebugResources'.
> Could not resolve all task dependencies for configuration ':app:debugRuntimeClasspath'.
   > Could not resolve io.grpc:grpc-core:[1.28.0].
     Required by:
         project :app > project :cloud_firestore > com.google.firebase:firebase-firestore:21.7.1 > io.grpc:grpc-android:1.28.0
         project :app > project :cloud_firestore > com.google.firebase:firebase-firestore:21.7.1 > io.grpc:grpc-okhttp:1.28.0
      > Failed to list versions for io.grpc:grpc-core.
         > Unable to load Maven meta-data from https://jcenter.bintray.com/io/grpc/grpc-core/maven-metadata.xml.
            > Could not HEAD 'https://jcenter.bintray.com/io/grpc/grpc-core/maven-metadata.xml'.
               > Read timed out

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org
BUILD FAILED in 1m 37s
Exception: Gradle task assembleDebug failed with exit code 1
Exited (sigterm)

这是我的build.gradle：

buildscript {
    ext.kotlin_version = '1.3.50'
    repositories {
        google()
        mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.6.2'
        classpath 'com.google.gms:google-services:4.3.3'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

allprojects {
    repositories {
        google()
        mavenCentral()
    }
}

rootProject.buildDir = '../build'
subprojects {
    project.buildDir = "${rootProject.buildDir}/${project.name}"
}
subprojects {
    project.evaluationDependsOn(':app')
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

是的，我的应用程序很旧，因为我还没有更新到零安全。
有什么提示可以避免将来出现此问题吗？
如何解决当前的问题？