Document attack_spec_version on collection objects

[isaisabel]

As a maintainer of ATT&CK, I want to document the current version of the ATT&CK Spec via collection bundles. This version number can be used to determine whether ATT&CK Workbench is capable of loading a given collection, or is too outdated to support its contents.

Update collections.md with x_mitre_attack_spec_version on collection objects.