Skip mono/nuget C# integration tests on Ubuntu 26.04

Ubuntu dropped the `mono-complete` package in 26.04, and mono's own apt
repository publishes nothing newer than `stable-focal`, so there is nothing to
fall back on. 25.04 and 25.10 still ship it, so 26.04 is the cutoff.

Without mono the legacy `packages.config` restore silently yields nothing and
the buildless extractor falls back to downloading .NET Framework reference
assemblies, so these four tests fail rather than being skipped.

Uses the new `runs_on` version comparisons, which also lets the macOS clause
collapse to `runs_on.macos < 15`.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

csharp/ql/integration-tests/posix/conftest.py

@@ -5,15 +5,10 @@ def _supports_mono_nuget():
     """
     Helper function to determine if the current platform supports Mono and nuget.
     
-    Returns True if running on Linux or on macOS x86_64 (excluding macos-15 and macos-26).
-    macOS ARM runners (macos-15 and macos-26) are excluded due to issues with Mono and nuget.
+    Returns True on Ubuntu before 26.04 and on macOS x86_64 before macOS 15. Linux other than
+    Ubuntu is not selected, as we do not test on it.
+    Ubuntu dropped the `mono-complete` package in 26.04, and mono is end-of-life, its own apt
+    repository publishing nothing newer than Ubuntu 20.04, so there is nothing to fall back on.
+    macOS 15 and later are ARM runners, which have issues with Mono and nuget.
     """
-    return (
-        runs_on.linux
-        or (
-            runs_on.macos
-            and runs_on.x86_64
-            and not runs_on.macos_15
-            and not runs_on.macos_26
-        )
-    )
+    return runs_on.ubuntu < 2604 or (runs_on.macos < 15 and runs_on.x86_64)