Set CA1507 to Warning and fix instances of errors (#7176)

* Set CA1507 to Warning and fix instances of errors

* Revert ruleset changes

* Change CA1507 to warning

Author: elachlan

eng/Common.globalconfig

@@ -200,7 +200,7 @@ dotnet_diagnostic.CA1505.severity = none
 dotnet_diagnostic.CA1506.severity = none
 
 # Use nameof to express symbol names
-dotnet_diagnostic.CA1507.severity = suggestion
+dotnet_diagnostic.CA1507.severity = warning
 
 # Avoid dead conditional code
 dotnet_diagnostic.CA1508.severity = none

src/Shared/ToolsetElement.cs

@@ -134,7 +134,7 @@ public string toolsVersion
         {
             get
             {
-                return (string)base["toolsVersion"];
+                return (string)base[nameof(toolsVersion)];
             }
 
             set
@@ -740,7 +740,7 @@ public string DefaultOverrideToolsVersion
                 // whenever the base class gives us an empty string.
                 // Note this means we can't distinguish between the attribute being present but containing
                 // an empty string for its value and the attribute not being present at all.
-                string defaultValue = (string)base["DefaultOverrideToolsVersion"];
+                string defaultValue = (string)base[nameof(DefaultOverrideToolsVersion)];
                 return String.IsNullOrEmpty(defaultValue) ? null : defaultValue;
             }