Update go1.21 and CI (#315)

* Update to go 1.21

Use `atomic.Bool` stdlib instead of including our own.

Include `tools\mkwinsyscall` updates from go-winio/283 to switch to
`syscallN`.
Note: removed `// TODO` about `print`/`ln`, since the latter adds spaces
between args when printing, which is undesired.

Also update CI to run steps on windows-2022 instead of windows-2019,
similar to our hcsshim CI.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

* PR: simplify type checking

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

---------

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

.golangci.yml

@@ -1,7 +1,3 @@
-run:
-  skip-dirs:
-    - pkg/etw/sample
-
 linters:
   enable:
     # style
@@ -24,6 +20,9 @@ linters:
     - unparam # unused function params
 
 issues:
+  exclude-dirs:
+    - pkg/etw/sample
+
   exclude-rules:
     # err is very often shadowed in nested scopes
     - linters:
@@ -70,9 +69,7 @@ linters-settings:
       # struct order is often for Win32 compat
       # also, ignore pointer bytes/GC issues for now until performance becomes an issue
       - fieldalignment
-    check-shadowing: true
   nolintlint:
-    allow-leading-space: false
     require-explanation: true
     require-specific: true
   revive: