fix(postgresflex): prevent state drifts on cron strings

[rubenhoenle]

Description

relates to STACKITSDK-413

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)

[github-actions]

Merging this branch changes the coverage (2 decrease, 2 increase)

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/mongodbflex/instance	29.76% (ø)
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/observability/instance	34.03% (ø)
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex/instance	21.13% (+0.39%)	👍
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/cluster	50.00% (ø)
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/sqlserverflex/instance	26.84% (ø)
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils	86.05% (+15.72%)	🎉
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/planmodifiers/int64planmodifier	46.67% (-40.83%)	💀 💀 💀 💀
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/planmodifiers/stringplanmodifier	50.00% (-37.50%)	💀 💀 💀

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/mongodbflex/instance/resource.go	33.72% (ø)	436	147	289
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/observability/instance/resource.go	37.55% (ø)	1004	377	627
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex/instance/resource.go	27.04% (+0.46%)	318 (+2)	86 (+2)	232	👍
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/cluster/resource.go	52.21% (ø)	881	460	421
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/sqlserverflex/instance/resource.go	31.13% (ø)	363	113	250
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/attributes.go	76.47% (+47.58%)	17 (-28)	13	4 (-28)	🌟
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/cron.go	100.00% (+100.00%)	9 (+9)	9 (+9)	0	🌟
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/planmodifiers/int64planmodifier/use_state_for_unknown_if.go	46.67% (-40.83%)	30 (+14)	14	16 (+14)	💀 💀 💀 💀
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/planmodifiers/stringplanmodifier/cron.go	62.50% (+62.50%)	8 (+8)	5 (+5)	3 (+3)	🌟
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/planmodifiers/stringplanmodifier/use_state_for_unknown_if.go	46.67% (-40.83%)	30 (+14)	14	16 (+14)	💀 💀 💀 💀
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/utils.go	82.50% (-1.41%)	80 (-7)	66 (-7)	14	👎

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.

Changed unit test files

• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex/instance/resource_test.go
• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/cron_test.go
• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/planmodifiers/stringplanmodifier/cron_test.go
• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/utils/utils_test.go