Bump CEL_SPEC_VERSION to v0.25.1 (#389)

Ref: https://buf.build/google/cel-spec/docs/v0.25.1

Author: stefanvanburen

Makefile

@@ -18,8 +18,9 @@ ADD_LICENSE_HEADER := $(BIN)/license-header \
 # This version should be kept in sync with the version in buf.yaml
 PROTOVALIDATE_VERSION ?= v1.0.0
 # Version of the cel-spec that this implementation is conformant with
-CEL_SPEC_VERSION ?= v0.24.0
-TESTDATA_FILE := test/testdata/string_ext.textproto
+# This should be kept in sync with the version in test/test_format.py
+CEL_SPEC_VERSION ?= v0.25.1
+TESTDATA_FILE := test/testdata/string_ext_$(CEL_SPEC_VERSION).textproto
 
 .PHONY: help
 help: ## Describe useful make targets