Spring Boot 2.3 系のときは id "org.springframework.boot" version "2.3.0.RELEASE" みたいに RELEASE が付いてた気がするけど、最新版は id "org.springframework.boot" version "2.4.0" になってる。
”The length can be specified as a value from 0 to 65,535. The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.”