Migrate solutions from .sln to .slnx format

Replaced traditional .sln files with simplified .slnx files for all solutions, including:
- Codebreaker.Analyzers
- Codebreaker.Backend.Cosmos
- Codebreaker.Backend.Models
- Codebreaker.Backend.Postgres
- Codebreaker.Backend.SqlServer
- Codebreaker.GameAPIs.Client

The .slnx format provides a cleaner, XML-based structure with `<Solution>` and `<Project>` elements, omitting detailed configuration and platform settings. This change improves simplicity, maintainability, and compatibility with modern tooling.

Author: christiannagel

src/Codebreaker.Analyzers.sln

@@ -0,0 +1,4 @@
+<Solution>
+  <Project Path="services/common/Codebreaker.GameAPIs.Analyzers.Tests/Codebreaker.GameAPIs.Analyzers.Tests.csproj" />
+  <Project Path="services/common/Codebreaker.GameAPIs.Analyzers/Codebreaker.Analyzers.csproj" />
+</Solution>

src/Codebreaker.Analyzers.slnx

@@ -0,0 +1,3 @@
+<Solution>
+  <Project Path="services/common/Codebreaker.Data.Cosmos/Codebreaker.Data.Cosmos.csproj" />
+</Solution>

src/Codebreaker.Backend.Cosmos.sln

@@ -0,0 +1,4 @@
+<Solution>
+  <Project Path="services/common/Codebreaker.GameAPIs.Models.Tests/Codebreaker.GameAPIs.Models.Tests.csproj" />
+  <Project Path="services/common/Codebreaker.GameAPIs.Models/Codebreaker.GameAPIs.Models.csproj" />
+</Solution>

src/Codebreaker.Backend.Cosmos.slnx

@@ -0,0 +1,4 @@
+<Solution>
+  <Folder Name="/Solution Items/" />
+  <Project Path="services/common/Codebreaker.Data.Postgres/Codebreaker.Data.Postgres.csproj" />
+</Solution>

src/Codebreaker.Backend.Models.sln

@@ -0,0 +1,4 @@
+<Solution>
+  <Project Path="services/common/Codebreaker.Data.SqlServer.Tests/Codebreaker.Data.SqlServer.Tests.csproj" />
+  <Project Path="services/common/Codebreaker.Data.SqlServer/Codebreaker.Data.SqlServer.csproj" />
+</Solution>