[FR]: Remove ObjC in documentation guides

### Description

This FR describes the deprecation and removal of Objective-C in Firebase's documentation guides only, excluding other areas of Firebase's documentation.

### API Proposal

Based on a sample of the Firestore guides, ObjC snippet selection is at ~17% of total Apple platforms snippet selection, and as Swift adoption continues we expect this value to go down further. Most ObjC developers are also capable of reading Swift, so where ObjC snippets are used as a reference ObjC developers are not necessarily hampered by their absence. Some guides, such as the getting started guides, are designed to help devs set up new projects, so it seems reasonable to assume most developers using those guides are setting up Swift projects. Removing the necessity of adding new ObjC snippets for every client-facing feature may allow us to focus more on snippets for new technologies like SwiftUI.

Currently, we're aiming to stop the addition of new ObjC snippets to Firebase documentation guides soon and then remove old snippets from those guides later on. Objective-C will continue to be fully supported in the SDKs and reference documentation.

Please let us know if this will negatively impact you.

### Firebase Product(s)

Documentation

The issue names no files, guides, or tests; start by locating Firebase documentation guides that contain Objective-C snippets and confirm that reference documentation and other Firebase documentation areas are excluded. Done means stopping new Objective-C guide snippets and, in the later phase, removing existing ones while preserving Swift guidance and SDK support.

Written by the indexing model from the issue text.