fix(postgresflex): fix backup schedule value used in acc tests

[GokceGK]

Description

relates to STACKITTPR-620

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 will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex	0.00% (ø)

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Coverage by file

Changed unit test files

• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/postgresflex/postgresflex_acc_test.go