Exclude Directory.Build.rsp from .gitignore template (#42401)

Co-authored-by: Chet Husk <[email protected]>

Author: MattKotsenas

template_feed/Microsoft.DotNet.Common.ItemTemplates/content/Gitignore/.gitignore

@@ -88,6 +88,8 @@ StyleCopReport.xml
 *.pgc
 *.pgd
 *.rsp
+# but not Directory.Build.rsp, as it configures directory-level build defaults
+!Directory.Build.rsp
 *.sbr
 *.tlb
 *.tli

test/dotnet-new.Tests/Approvals/AllCommonItemsCreate.-o#dotnet-gitignore-file#-n#item.verified/dotnet-gitignore-file/.gitignore

@@ -88,6 +88,8 @@ StyleCopReport.xml
 *.pgc
 *.pgd
 *.rsp
+# but not Directory.Build.rsp, as it configures directory-level build defaults
+!Directory.Build.rsp
 *.sbr
 *.tlb
 *.tli