refactor(core): use const for descriptor in `LegacyMigrationResolver`

[trivikr]

What's the problem this PR addresses?

The variable descriptor can be declared as const, and assigned values in a conditional operator. Another const can be used for initial parsed descriptor.

How did you fix it?

Use const for variable descriptor, and introduce new const parsedDescriptor

Checklist

• [x] I have read the Contributing Guide.

• [x] I have set the packages that need to be released for my changes to be effective.

• [x] I will check that all automated PR checks pass before the PR gets reviewed.