config: migrate gomodguard to gomodguard_v2 in canonical .golangci.yml

golangci-lint v2.12.0 deprecated `gomodguard` in favour of `gomodguard_v2`.
Because the canonical config uses `default: all`, the deprecated linter is
auto-enabled and every lint run in every consuming Go repo emits a
deprecation warning.

`gomodguard` and `gomodguard_v2` are two independently registered linters
in v2.12.2, so `gomodguard_v2` is already enabled by `default: all`; only
the deprecated one needs disabling. This mirrors the existing `wsl` entry,
which was disabled for the same reason when `wsl_v5` superseded it.

No settings migration was required: the canonical config configures no
gomodguard settings at all.

Refs: #25

.golangci.yml

@@ -16,6 +16,7 @@ linters:
     - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wsl              # Deprecated, replaced by wsl_v5
+    - gomodguard       # Deprecated, replaced by gomodguard_v2
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
   settings: