ref(types): Move SeverityLevel and SeverityLevels to @sentry/utils

[lobsterkatie]

This PR includes three changes:

• It moves the type SeverityLevel and the const string array SeverityLevels from @sentry/types to @sentry/utils, in order that only types get exported from @sentry/types. Since every package other than @sentry/types depends on @sentry/utils, they're still accessible to any package which needs them.

• It changes all of our uses of SeverityLevel and SeverityLevels so that they point to the copies in @sentry/utils rather than those in @sentry/types.

• It adds a corresponding entry to the migration guide.

The copies of SeverityLevel and SeverityLevels in @sentry/types remain, for backwards compatibility, but can be removed in v7.

[github-actions]

size-limit report

Path	Base Size (b6a6142)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	19.68 KB	19.68 KB	+0.02% 🔺
@sentry/browser - CDN Bundle (minified)	62.89 KB	62.9 KB	+0.01% 🔺
@sentry/browser - Webpack	22.21 KB	22.21 KB	-0.01% 🔽
@sentry/browser - Webpack - gzip = false	76.02 KB	76.02 KB	0%
@sentry/react - Webpack	22.24 KB	22.24 KB	-0.01% 🔽
@sentry/nextjs Client - Webpack	46.24 KB	46.24 KB	-0.01% 🔽
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.22 KB	28.23 KB	+0.02% 🔺