fixes #20426; remove maincommand and m options since they are a no op since 2014 (#20429)

* bump macOS image on Azure CI to macos-11

##[warning]The macOS-10.15 environment is deprecated, consider switching to macos-11(macos-latest), macos-12 instead. For more details see https://github.com/actions/virtual-environments/issues/5583

* fix CI error

* fixes #20426; remove `maincommand` and `m` options since they are a noop since 2014 and causes confusion

fixes #20426

7f7b13a45f (diff-d949f8c356fd2dc9ceedc6f3dbbd01e2c806269dd0a8ad6516facf589fa2c99a) makes it a no op, but it causes a regression because it should add `expectArg(switch, arg, pass, info)` before the discard statement. It causes https://github.com/nim-lang/Nim/issues/20426 to happen. Without `expectArg(switch, arg, pass, info)`, `-mm:orc` is wrongly interpreted as `-m` and compiler, which doesn't make sense. It should either abort compilation or prints `argument for command line option expected: '-m'` message. Since they are a no op since 2014, let's remove it to clear the confusion. Let's wait and see whether it breaks something.

* add a changelog

changelog.md

@@ -12,6 +12,8 @@
 
 - The `gc:v2` option is removed.
 
+- The `mainmodule` and `m` options are removed.
+
 - The `threads:on` option is now the default.
 
 - Optional parameters in combination with `: body` syntax (RFC #405) are now opt-in via

compiler/commands.nim

@@ -1094,7 +1094,7 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     processOnOffSwitchG(conf, {optEnableDeepCopy}, arg, pass, info)
   of "": # comes from "-" in for example: `nim c -r -` (gets stripped from -)
     handleStdinInput(conf)
-  of "nilseqs", "nilchecks", "mainmodule", "m", "symbol", "taintmode", "cs", "deadcodeelim": warningOptionNoop(switch)
+  of "nilseqs", "nilchecks", "symbol", "taintmode", "cs", "deadcodeelim": warningOptionNoop(switch)
   of "nimmainprefix": conf.nimMainPrefix = arg
   else:
     if strutils.find(switch, '.') >= 0: options.setConfigVar(conf, switch, arg)