`AllCops does not support TargetRailsVersion parameter` warning

[andy-rootly]

This issue was written with Claude Code's assistance.

Problem

When using rubocop 1.84.2 / rubocop-rails 2.34.3, running RuboCop produces:

Warning: AllCops does not support TargetRailsVersion parameter.

Cause

rubocop-rails handles this in its own plugin.rb by monkey-patching ConfigValidator::COMMON_PARAMS to include TargetRailsVersion:

# FIXME: This is a dirty hack relying on a private constant to prevent
# "Warning: AllCops does not support TargetRailsVersion parameter".
ConfigValidator.const_set(:COMMON_PARAMS, ConfigValidator::COMMON_PARAMS.dup << 'TargetRailsVersion')

But standard-rails bypasses rubocop-rails' normal plugin loading (via load_rubocop_rails_without_the_monkey_patch.rb), so that hack never runs.

PR #97 (released in v1.6.0) addressed this for MigratedSchemaVersion (unconditional delete) but only removes TargetRailsVersion when its value is nil:

all_cops.delete("MigratedSchemaVersion")
all_cops.delete("TargetRailsVersion") if all_cops.key?("TargetRailsVersion") && all_cops["TargetRailsVersion"].nil?

Since rubocop-rails' default.yml sets TargetRailsVersion: ~ (YAML nil), the conditional delete does match — but only before a user sets a target_rails_version in .standard.yml. In practice, the warning still appears (possibly due to config merging order or rubocop version differences).

Gem versions

rubocop (1.84.2)
rubocop-rails (2.34.3)
standard (1.54.0)
standard-rails (1.6.0)

Suggested fix

Unconditionally remove TargetRailsVersion from AllCops, matching the existing MigratedSchemaVersion handling:

all_cops.delete("MigratedSchemaVersion")
all_cops.delete("TargetRailsVersion")