Fix MSB4086 if LangVersion is a keyword (#1420)

Fix compilation error MSB4086 if LangVersion is already set to one of the keywords such as preview, latest and latestMajor. Otherwise MSBuild complains that it cannot perform a numeric comparison.

Author: martincostello

src/BenchmarkDotNet/Templates/CsProj.txt

@@ -17,7 +17,7 @@
     <CodeAnalysisRuleSet></CodeAnalysisRuleSet>
     $COPIEDSETTINGS$
     <!-- we set LangVersion after $COPIEDSETTINGS$ which might contain LangVersion copied from the benchmarks project -->
-    <LangVersion Condition="'$(LangVersion)' == '' Or '$(LangVersion)' &lt; '7.3'">latest</LangVersion>
+    <LangVersion Condition="'$(LangVersion)' == '' Or ($([System.Char]::IsDigit('$(LangVersion)', 0)) And '$(LangVersion)' &lt; '7.3')">latest</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>