modify plugin.xml

[dann95]

Autoident file and upgrade tags from "framework" to "pod", everytime i was trying to install, i received the message that "framework" was deprecated and will be removed soon, when i changed from the original plugin to this fork, the Podfile was always empty, and the build generates an error "could not find Firebase.h", upgrading to "pod" tag, it generates the Podfile normally and no build error. This may remove support to old versions, but at any time Cordova will finally remove the use of this tag.

[dann95]

generated file with new tags in my mac OS:

platform :ios, '9.0'
use_frameworks!
target 'Myappname' do
	pod 'Firebase/Core', '5.15.0'
	pod 'Firebase/Auth', '5.15.0'
	pod 'Firebase/Messaging', '5.15.0'
	pod 'Firebase/Performance', '5.15.0'
	pod 'Firebase/RemoteConfig', '5.15.0'
	pod 'Fabric', '1.9.0'
	pod 'Crashlytics', '3.12.0'
end

generated file with previous tags in my mac OS:

platform :ios, '7.0'
target 'Myappname' do
end

[dpa99c]

The <framework type="podspec"> is from the original PR which I merged to this fork (since it hasn't been merged to main cordova-plugin-firebase).
So replacing it with <pod> is fine since our fork of cordova-plugin-cocoapod-support supports it and parses it.
So this is good to merge.