gfe-relnote: Change a string to a char[16] to avoid using data() in string, which is sometimes const. Fixes a Chrome merge failure of cr/305944808. Protected by existing flags gfe2_reloadable_flag_quic_enable_version_draft_25_v3 and gfe2_reloadable_flag_quic_enable_version_draft_27. Non-const data() on string is available in C++17. PiperOrigin-RevId: 306270586 Change-Id: I15a42c3928bb413dbc1ab3a2adde2f5a339df027