Document how to upgrade an Eclipse ESCET development environment without a reinstall

Currently, when upgrading to a new Eclipse version, and to a lesser degree when upgrading to a new Java version, we require a fresh new development environment to be set up. However, that is not necessary. In both cases, an in-place update is possible.

This was mentioned shortly at !399 (comment 1033749). I recently tried this for a colleague who created an Eclipse ESCET development environment using a too old Eclipse, and the in-place updating of Eclipse worked well.

I think documenting this would help, as it makes it easier to migrate.