fix(postgresflex): fix state drift when migrating from v2 to v3

[SerseusWasTaken]

Description

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[SerseusWasTaken]

We should release stackitcloud/stackit-cli#1505 first before releasing this fix

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex/instance	28.00% (ø)
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/sqlserverflex/instance	33.91% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex/instance/resource.go	33.18% (ø)	422	140	282
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/sqlserverflex/instance/resource.go	37.61% (ø)	468	176	292

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.