Bump version to 5.100

<manifest

    package="com.fsck.k9"

    xmlns:android="http://schemas.android.com/apk/res/android"

    android:versionCode="21008"

    android:versionName="4.905">

    android:versionCode="23000"

    android:versionName="5.100">

    <uses-sdk

        android:minSdkVersion="15"