Fix LanguageVersion for perf tests (Azure#46874)

Author: weshaggard

eng/pipelines/templates/jobs/perf.yml

@@ -1,7 +1,8 @@
 parameters:
+# LanguageVersion in context of .NET perf tests corresponds to the .NET framework version and is set in individual perf.yml files
 - name: LanguageVersion
   type: string
-  default: '7'
+  default: ''
 - name: ServiceDirectory
   type: string
   default: ''

sdk/monitor/perf.yml

@@ -1,8 +1,8 @@
 parameters:
 - name: LanguageVersion
-  displayName: LanguageVersion (6, 7)
+  displayName: LanguageVersion
   type: string
-  default: '7'
+  default: '8'
 - name: PackageVersions
   displayName: PackageVersions (regex of package versions to run)
   type: string