CI: Test EUnit only with Erlang 26 or higher

Three tests fail with older Erlang versions:
  fxml_test: splitted_stream_test...*failed*
  fxml_test: streaming_mismatched_tags_error_test...*failed*
  fxml_test: streaming_invalid_attribute_error_test...*failed*

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
     - run: rebar3 xref
     - run: rebar3 dialyzer
     - run: rebar3 eunit -v
-      if: matrix.otp >= 24
+      if: matrix.otp >= 26
 
   cover:
     name: Cover