fix: make plan non-nullable and drop plan type

[ajay-sentry]

This PR is an extension of codecov/worker#1103, closing the loop on plan type thats nullable as well as removing the default value for the plan which was a plan name we don't want to support anymore.

This will hopefully prevent others from falling into the same pitfall we did

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. In 2022 this entity acquired Codecov and as result Sentry is going to need some rights from me in order to utilize my contributions in this PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[codspeed-hq]

CodSpeed Performance Report

Merging #533 will create unknown performance changes

_{Comparing Ajay/drop-nullable-plan-and-default-type (c171275) with main (47fa7fd)}

Summary

⚠️ No benchmarks were detected in both the base of the PR and the PR.\

[adrian-codecov]

Pending confirmation if these should be risky changes and what is the effect from making a plan non-nullable