Adjust README to show status badges from GitHub Actions workflows (#1364)

This adjusts `README.md` to begin showing CI status badges from the
GitHub Actions workflows I configured in #1361.

View [rendered README
document](https://github.com/stmontgomery/swift-testing/blob/github-actions-badges/README.md).

### Motivation:

This will help ensure our CI status badges remain consistent with the CI
status shown on Pull Requests.

### Modifications:

- Remove the CI status column on the "Supported platforms" table from
the README.
- This is because the new workflows do not offer granular, per-platform
status badges (see [GitHub's
documentation](https://docs.github.com/en/actions/how-tos/monitor-workflows/add-a-status-badge)).
- Add a new section to the README showing this status.
- Drive-by improvement: Move "Windows" platform up a few rows so that
all the Supported platforms are listed first, followed by all
Experimental platforms.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

README.md

@@ -13,6 +13,9 @@ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 Swift Testing is a package with expressive and intuitive APIs that make testing
 your Swift code a breeze.
 
+[![CI status badge for main branch using main toolchain](https://github.com/swiftlang/swift-testing/actions/workflows/main_using_main.yml/badge.svg?branch=main&event=push)](https://github.com/swiftlang/swift-testing/actions/workflows/main_using_main.yml)
+[![CI status badge for main branch using 6.2 toolchain](https://github.com/swiftlang/swift-testing/actions/workflows/main_using_release.yml/badge.svg?branch=main&event=push)](https://github.com/swiftlang/swift-testing/actions/workflows/main_using_release.yml)
+
 ## Feature overview
 
 ### Clear, expressive API
@@ -103,18 +106,14 @@ Swift.
 The table below describes the current level of support that Swift Testing has
 for various platforms:
 
-| **Platform** | **CI Status (6.2)** | **CI Status (main)** | **Support Status** |
-|---|:-:|:-:|---|
-| **macOS** | [![Build Status](https://ci.swift.org/buildStatus/icon?job=swift-testing-main-swift-6.2-macos)](https://ci.swift.org/job/swift-testing-main-swift-6.2-macos/) | [![Build Status](https://ci.swift.org/buildStatus/icon?job=swift-testing-main-swift-main-macos)](https://ci.swift.org/view/Swift%20Packages/job/swift-testing-main-swift-main-macos/) | Supported |
-| **iOS** | | | Supported |
-| **watchOS** | | | Supported |
-| **tvOS** | | | Supported |
-| **visionOS** | | | Supported |
-| **Ubuntu 22.04** | [![Build Status](https://ci.swift.org/buildStatus/icon?job=swift-testing-main-swift-6.2-linux)](https://ci.swift.org/job/swift-testing-main-swift-6.2-linux/) | [![Build Status](https://ci.swift.org/buildStatus/icon?job=swift-testing-main-swift-main-linux)](https://ci.swift.org/view/Swift%20Packages/job/swift-testing-main-swift-main-linux/) | Supported |
-| **FreeBSD** | | | Experimental |
-| **OpenBSD** | | | Experimental |
-| **Windows** | [![Build Status](https://ci-external.swift.org/buildStatus/icon?job=swift-testing-main-swift-6.2-windows)](https://ci-external.swift.org/view/all/job/swift-testing-main-swift-6.2-windows/) | [![Build Status](https://ci-external.swift.org/buildStatus/icon?job=swift-testing-main-swift-main-windows)](https://ci-external.swift.org/job/swift-testing-main-swift-main-windows/) | Supported |
-| **Wasm** | | | Experimental |
+| **Platform** | **Support Status** |
+|-|-|
+| Apple platforms | Supported |
+| Linux | Supported |
+| Windows | Supported |
+| FreeBSD, OpenBSD | Experimental |
+| Wasm | Experimental |
+| Android | Experimental |
 
 ### Works with XCTest