NCBC-4132: Fix build command

Motivation
----------
The GHA build command was wrongly passing /p:TargetFrameworkVersion=v4.8.

Changes
-------
Build only with NET48 by specifying the build target, but not the SDK version.

Change-Id: I8147d5e09e33aed0209251f482d426353c08981f
Reviewed-on: https://review.couchbase.org/c/couchbase-net-client/+/239454
Tested-by: Emilien Bevierre <emilien.bevierre@couchbase.com>
Reviewed-by: Emilien Bevierre <emilien.bevierre@couchbase.com>

Author: emilienbev

.github/workflows/build-and-test-net48.yml

@@ -26,7 +26,7 @@ jobs:
         run: nuget restore couchbase-net-client.sln
 
       - name: Build (Release)
-        run: msbuild couchbase-net-client.sln /p:Configuration=Release /p:TargetFrameworkVersion=v4.8 /m
+        run: msbuild couchbase-net-client.sln /p:Configuration=Release /p:TargetFramework=net48 /m
 
       - name: Setup VSTest
         uses: darenm/Setup-VSTest@v1.3