Add member months to core

[thutuva]

Describe your changes

Please include a summary of any changes.

• Moved the model member_months to the Core data mart. This table will materialize as core.member_months.
• Update refs and yaml files

How has this been tested?

Please describe the tests you ran to verify your changes. Provide instructions or code to reproduce output.
manual dbt build completed successfully

Reviewer focus

Please summarize the specific items you’d like the reviewer(s) to look into.
n/a

Checklist before requesting a review

• I have added at least one Github label to this PR (bug, enhancement, breaking change,...)
• My code follows style guidelines
• (New models) YAML files are categorized by sub folder and models listed in alphabetical order
• (New models) I have added a config to each new model to enable it for claims and/or clinical data
• (New models) I have added the variable tuva_last_run to the final output
• (Optional) I have recorded a Loom to explain this PR

Package release checklist

• I have updated dbt docs
• I have updated the version number in the dbt_project.yml

(Optional) Gif of how this PR makes you feel

Loom link

https://www.loom.com/share/08c6d743418d4375b4e53dc2a0eb9664?sid=eabd9c9e-785d-4414-9b7b-f7f70d83eab5