docs: add maltesmann as a contributor for code

[allcontributors]

Adds @maltesmann as a contributor for code.

This was requested by Tenischev in this comment

[skip ci]

[derberg]

@Tenischev there is a problem. For some reason PR testing workflows are not set as mandatory to merge a PR

so this went through: #424 but it should not, as it is breaking tests, as it is bumping generator version that this template is not configured to use.

instead of downgrading I suggest you open a PR to update https://github.com/asyncapi/java-spring-template/blob/master/package.json#L122 value. It will basically mean that we shoudl have major as new generator dropped support for old nodejs versions