Only allow allow squash merging

[dbeatty10]

Describe the under the hood change

Currently, there are two options enabled for merging PRs in dbt-utils:

• squash merging
• rebase merging

But we should update it to align with dbt-core, et al:

Who will this benefit?

This will benefit maintainers -- lower decision fatigue by only giving a single option.
This will also prevent rebase merges which will write the commit history differently and maybe less readable.

[dbeatty10]

Done by updating the settings.

🎩

Working as expected: