Upgrade to Spoon Core 9.1.0 and Spoon Maven Plugin 3.5

We're currently using Spoon 7.x, so the move to Spoon 9.x will probably need some work. Note that Spoon 9.1.0 drop support for Java 8

See:

• https://github.com/INRIA/spoon/pull/4104 (Spoon 9.1.0 release notes)
• https://github.com/INRIA/spoon/issues/3845 (for Spoon 9 release notes)
• https://github.com/INRIA/spoon/releases/tag/spoon-core-8.0.0
• https://github.com/INRIA/spoon/pull/3310
• https://github.com/INRIA/spoon/pull/3501
• https://github.com/INRIA/spoon/pull/3647