Makefile.common: Add check for future copyright years

Add validation in common-check_license to detect and reject copyright
headers with years 2026 or later. This enforces the removal of copyright
dates as per https://github.com/prometheus/proposals/blob/main/proposals/0050-remove-copyright-dates.md

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

Makefile.common

@@ -129,6 +129,12 @@ common-check_license:
                echo "license header checking failed:"; echo "$${licRes}"; \
                exit 1; \
        fi
+	@echo ">> checking for copyright years 2026 or later"
+	@futureYearRes=$$(git grep -E 'Copyright (202[6-9]|20[3-9][0-9])' -- '*.go' ':!:vendor/*' || true); \
+	if [ -n "$${futureYearRes}" ]; then \
+		echo "Files with copyright year 2026 or later found (should use 'Copyright The Prometheus Authors'):"; echo "$${futureYearRes}"; \
+		exit 1; \
+	fi
 
 .PHONY: common-deps
 common-deps: