Makefile: Fix awk command in "make help"

Typing "make help" currently prints a waring from awk (GNU Awk 5.1.0):

    $ make help

    Usage:
      make <target>

    Targets:
    awk: cmd. line:1: warning: regexp escape sequence `\_' is not a known regexp operator
      pwru                 Build the GO binary
      release              Build the GO binary within a Docker container
      local-release        Build a new release
      install              Install the GO Binary to the location specified by 'BINDIR'
      clean                Clean up build artifacts
      test                 Run GO tests
      help                 Show this hel

This is because the Makefile escapes the underscore ("\_"). As far as I
know, there is no reason to escape it, so we can just remove the
backslash. In the same list of characters, there's no reason to escape
the dash either, the man page for GNU Awk says: "To include a literal
dash in the list, put it first or last", so we can just do that.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Makefile

@@ -79,7 +79,7 @@ help:
 	@echo ''
 	@echo 'Targets:'
 
-	@awk '/^[a-zA-Z\-\_0-9]+:/ { \
+	@awk '/^[a-zA-Z0-9_-]+:/ { \
 		helpMessage = match(lastLine, /^## (.*)/); \
 		if (helpMessage) { \
 			helpCommand = substr($$1, 0, index($$1, ":")-1); \