SpringBoot Dependency management for mimepull is redundant and the managed version is incompatible with Java 8

Mimepull dependency was introduced in 1c18fd8 for gh-14924 to force the version as the one coming from saaj-impl was not on Maven Central. This is no longer the case.

Version 1.10.0 of Mimepull has been built with Java 11, breaking compatibility with Java 8 for Spring Boot.

Closes gh-31133

Comment From: wilkinsona

Thanks very much for making your first contribution to Spring Boot, @youribonnaffe.