Switch to plain main string processing branch

With the StringProcessing library long out of initial development,
we don't need the extra step of maintaining a separate `swift/main`
branch. This switches to use the `main` branch of the StringProcessing
project for building the `main` branch of the stdlib. The release
branch name strategy is unchanged.

Author: natecook1000

utils/update_checkout/update-checkout-config.json

@@ -173,7 +173,7 @@
                 "swift-docc-symbolkit": "main",
                 "swift-markdown": "main",
                 "swift-nio": "2.65.0",
-                "swift-experimental-string-processing": "swift/main",
+                "swift-experimental-string-processing": "main",
                 "swift-sdk-generator": "main",
                 "wasi-libc": "wasi-sdk-27",
                 "wasmkit": "0.1.6",
@@ -601,7 +601,7 @@
                 "swift-docc-symbolkit": "main",
                 "swift-markdown": "main",
                 "swift-nio": "2.65.0",
-                "swift-experimental-string-processing": "swift/main",
+                "swift-experimental-string-processing": "main",
                 "swift-sdk-generator": "main",
                 "wasi-libc": "wasi-sdk-27",
                 "wasmkit": "0.1.6",
@@ -659,7 +659,7 @@
                 "swift-docc-symbolkit": "main",
                 "swift-markdown": "main",
                 "swift-nio": "2.65.0",
-                "swift-experimental-string-processing": "swift/main",
+                "swift-experimental-string-processing": "main",
                 "swift-sdk-generator": "main",
                 "wasi-libc": "wasi-sdk-27",
                 "wasmkit": "0.1.6",