In order to overcome this issue.
spring-projects/spring-boot#11225

Also had to pull back jaxb-runtime, as this is now pinned by hibernate.

We were previously creating 3 nested retry loops, first in
DuraStoreSyncEndpoint, then again in ContentIterator, ultimately
calling the `getSpace` method, which is already a `Retriable`! We
now solely rely on the retry loop created by `ContentStore.getSpace()`.

Reverted ContentIterator.

Switched out the call in spaceExists() to use getSpaceACLs() instead of
getSpaceContents() so it doesn't end up in a double loop when the space
does not exist (and therefore returns an error faster).

There is no need to do this because the return value is never used.

When running in command-line mode, it is too easy to mis-type the space
name. Next thing you know, you've got tons of content in the incorrect
space. Synctool will now error if the space does not exist.