Rename spring-session.bom.version property to spring-session.version to align with changes to Spring Session's version scheme and bom

[wilkinsona]

All of Spring Sesson's modules are now part of its core repository and, as a result, its bom has switched from calver to a version that aligns with the core. Furthermore, the bom has moved into the core repository. Given these changes, it no longer makes sense for us to be managing a version for the bom specifically, it should just be for Spring Session in general. This will result in the version property changing from spring-session-bom.version to spring-session.version.