Drop compatibility with 6.0.0 (#4411)

**What type of PR is this?**

Cleanup

**What does this PR do? Why is it needed?**

Simplify C++ toolchain handling by requiring at least Bazel 6.1.0. This
will be used in future PRs to implement proper, exec-platform aware
resolution for the combination of Go and (optional) C++ toolchain.

**Which issues(s) does this PR fix?**

**Other notes for review**

Author: fmeum

go/private/BUILD.bazel

@@ -64,7 +64,6 @@ bzl_library(
         "@bazel_skylib//rules:common_settings",
         "@bazel_tools//tools/build_defs/cc:action_names.bzl",
         "@bazel_tools//tools/cpp:toolchain_utils.bzl",
-        "@io_bazel_rules_go_bazel_features//:features",
         "@io_bazel_rules_nogo//:scope.bzl",
     ],
 )

go/private/context.bzl

@@ -34,7 +34,6 @@ load(
     "@bazel_tools//tools/cpp:toolchain_utils.bzl",
     "find_cpp_toolchain",
 )
-load("@io_bazel_rules_go_bazel_features//:features.bzl", "bazel_features")
 load(
     "@io_bazel_rules_nogo//:scope.bzl",
     NOGO_EXCLUDES = "EXCLUDES",
@@ -707,10 +706,7 @@ def _cgo_context_data_impl(ctx):
     # toolchain (to be inputs into actions that need it).
     # ctx.files._cc_toolchain won't work when cc toolchain resolution
     # is switched on.
-    if bazel_features.cc.find_cpp_toolchain_has_mandatory_param:
-        cc_toolchain = find_cpp_toolchain(ctx, mandatory = False)
-    else:
-        cc_toolchain = find_cpp_toolchain(ctx)
+    cc_toolchain = find_cpp_toolchain(ctx, mandatory = False)
     if not cc_toolchain or cc_toolchain.compiler in _UNSUPPORTED_C_COMPILERS:
         return []
 
@@ -924,7 +920,7 @@ def _cgo_context_data_impl(ctx):
 cgo_context_data = rule(
     implementation = _cgo_context_data_impl,
     attrs = {
-        "_cc_toolchain": attr.label(default = "@bazel_tools//tools/cpp:optional_current_cc_toolchain" if bazel_features.cc.find_cpp_toolchain_has_mandatory_param else "@bazel_tools//tools/cpp:current_cc_toolchain"),
+        "_cc_toolchain": attr.label(default = "@bazel_tools//tools/cpp:optional_current_cc_toolchain"),
         "_xcode_config": attr.label(
             default = "@bazel_tools//tools/osx:current_xcode_config",
         ),
@@ -933,8 +929,7 @@ cgo_context_data = rule(
         # In pure mode, a C++ toolchain isn't needed when transitioning.
         # But if we declare a mandatory toolchain dependency here, a cross-compiling C++ toolchain is required at toolchain resolution time.
         # So we make this toolchain dependency optional, so that it's only attempted to be looked up if it's actually needed.
-        # Optional toolchain support was added in bazel 6.0.0.
-        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False) if hasattr(config_common, "toolchain_type") else "@bazel_tools//tools/cpp:toolchain_type",
+        config_common.toolchain_type("@bazel_tools//tools/cpp:toolchain_type", mandatory = False),
     ],
     fragments = ["apple", "cpp"],
     doc = """Collects information about the C/C++ toolchain. The C/C++ toolchain

go/private/polyfill_bazel_features.bzl

@@ -5,9 +5,6 @@
 # We just implement the checks we've seen we actually need, and hope to delete this completely when we are in a pure-bzlmod world.
 
 _POLYFILL_BAZEL_FEATURES = """bazel_features = struct(
-  cc = struct(
-    find_cpp_toolchain_has_mandatory_param = {find_cpp_toolchain_has_mandatory_param},
-  ),
   external_deps = struct(
     # WORKSPACE users have no use for bazel mod tidy.
     bazel_mod_tidy = False,
@@ -16,16 +13,6 @@ _POLYFILL_BAZEL_FEATURES = """bazel_features = struct(
 """
 
 def _polyfill_bazel_features_impl(rctx):
-    # An empty string is treated as a "dev version", which is greater than anything.
-    bazel_version = native.bazel_version or "999999.999999.999999"
-    version_parts = bazel_version.split("-")[0].split(".")
-    if len(version_parts) != 3:
-        fail("invalid Bazel version '{}': got {} dot-separated segments, want 3".format(bazel_version, len(version_parts)))
-    major_version_int = int(version_parts[0])
-    minor_version_int = int(version_parts[1])
-
-    find_cpp_toolchain_has_mandatory_param = major_version_int > 6 or (major_version_int == 6 and minor_version_int >= 1)
-
     rctx.file("BUILD.bazel", """
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 bzl_library(
@@ -35,9 +22,7 @@ bzl_library(
 )
 exports_files(["features.bzl"])
 """)
-    rctx.file("features.bzl", _POLYFILL_BAZEL_FEATURES.format(
-        find_cpp_toolchain_has_mandatory_param = repr(find_cpp_toolchain_has_mandatory_param),
-    ))
+    rctx.file("features.bzl", _POLYFILL_BAZEL_FEATURES)
 
 polyfill_bazel_features = repository_rule(
     implementation = _polyfill_bazel_features_impl,