【エラー解消】 pod install, flutterfire, firestore

flutterでfirestoreをinstall後に、突然エラーがでて苦しまされた。。

以下、その時のエラー文を一部抜粋

 Automatically assigning platform `iOS` with version `9.0` on target
    `Runner` because no platform was specified. Please specify a platform for
    this target in your Podfile

Inspecting targets to integrate
      Using `ARCHS` setting to build architectures of target `Pods-Runner`: (``)

    Fetching external sources
    -> Fetching podspec for `Flutter` from `Flutter`
    -> Fetching podspec for `cloud_firestore` from
    `.symlinks/plugins/cloud_firestore/ios`
    cloud_firestore: Using Firebase SDK version '8.15.0' defined in
    'firebase_core'
    -> Fetching podspec for `firebase_auth` from
    `.symlinks/plugins/firebase_auth/ios`
    firebase_auth: Using Firebase SDK version '8.15.0' defined in
    'firebase_core'
    -> Fetching podspec for `firebase_core` from
    `.symlinks/plugins/firebase_core/ios`
    firebase_core: Using Firebase SDK version '8.15.0' defined in
    'firebase_core'

    Resolving dependencies of `Podfile`
      CDN: trunk Relative path: CocoaPods-version.yml exists! Returning local
      because checking is only performed in repo update
    [!] CocoaPods could not find compatible versions for pod "cloud_firestore":
      In Podfile:
        cloud_firestore (from `.symlinks/plugins/cloud_firestore/ios`)

    Specs satisfying the `cloud_firestore (from
    `.symlinks/plugins/cloud_firestore/ios`)` dependency were found, but they
    required a higher minimum deployment target.

＜解決＞
１. Podfile.lock を削除
２. Podfile の2行目にコメントアウトされている`# platform :ios, '9.0'`のコメントを外し、`platform :ios, '11.0'`へ変更

この結果、無事buildできるようになりました。
※ buildの際ものすごく時間がかかりますが、何もせず待ち続けました。

正確な原因不明ですが、ひとまず復活！

＜参考＞

CocoaPods could not find compatible versions for pod "Firebase/Core” | cloud_firestore, Flutter