[git] Re-format .gitmodules (#8978)

Context: 059c2c0

Commits a7b5768 and 6d2a4bb added submodules and updated
`.gitmodules` in a manner *not* in accordance with 059c2c0:

  * They used tabs instead of spaces, which means @MSylvia's branch
    tooling will cause larger commit diffs for this file

  * The entries weren't sorted.

  * Branch names weren't specified.

Reformat the entries within `.gitmodules`, sorting the names,
replacing tabs with spaces, and ensuring that all submodules have
a branch name specified.

Author: grendello

.gitmodules

@@ -2,6 +2,10 @@
     path = external/android-api-docs
     url = https://github.com/xamarin/android-api-docs
     branch = main
+[submodule "external/constexpr-xxh3"]
+    path = external/constexpr-xxh3
+    url = https://github.com/chys87/constexpr-xxh3.git
+    branch = main
 [submodule "external/debugger-libs"]
     path = external/debugger-libs
     url = https://github.com/mono/debugger-libs
@@ -10,6 +14,10 @@
     path = external/Java.Interop
     url = https://github.com/xamarin/java.interop.git
     branch = main
+[submodule "external/libunwind"]
+    path = external/libunwind
+    url = https://github.com/libunwind/libunwind.git
+    branch = v1.8-stable
 [submodule "external/lz4"]
     path = external/lz4
     url = https://github.com/xamarin/lz4
@@ -23,12 +31,6 @@
     url = https://github.com/xamarin/xamarin-android-tools
     branch = main
 [submodule "external/xxHash"]
-	path = external/xxHash
-	url = https://github.com/Cyan4973/xxHash.git
-[submodule "external/constexpr-xxh3"]
-	path = external/constexpr-xxh3
-	url = https://github.com/chys87/constexpr-xxh3.git
-[submodule "external/libunwind"]
-	path = external/libunwind
-	url = https://github.com/libunwind/libunwind.git
-	branch = master
+    path = external/xxHash
+    url = https://github.com/Cyan4973/xxHash.git
+    branch = release