chore: separate go minimum and build toolchain (#38559)

- split the go version in `go.mod` into `go` and `toolchain` again
- add a workaround for https://github.com/golang/go/issues/75331 so
`make tidy` never drops `toolchain`
- configure renovate to bump `go` on minor releases and `toolchain` on
every release
- go and toolchain bumps land in a separate, fast-tracked PR, as both
carry security fixes

Replaces: https://github.com/go-gitea/gitea/pull/37846

Makefile

@@ -426,7 +426,12 @@ unit-test-coverage:
 .PHONY: tidy
 tidy: ## run go mod tidy
 	$(eval MIN_GO_VERSION := $(shell grep -Eo '^go\s+[0-9]+\.[0-9.]+' go.mod | cut -d' ' -f2))
+	$(eval GO_TOOLCHAIN := $(shell grep -Eo '^toolchain\s+go[0-9.]+' go.mod | cut -d' ' -f2))
 	$(GO) mod tidy -compat=$(MIN_GO_VERSION)
+	@# workaround https://github.com/golang/go/issues/75331: restore toolchain if tidy dropped it
+	@if [ -n "$(GO_TOOLCHAIN)" ] && ! grep -qE '^toolchain\s' go.mod; then \
+		$(GO) mod edit -toolchain=$(GO_TOOLCHAIN); \
+	fi
 	@$(MAKE) --no-print-directory $(GO_LICENSE_FILE)
 
 vendor: go.mod go.sum