[Backport 1.8.latest] Fix setting `silence` of `warn_error_options` via `dbt_project.yaml` flags

[github-actions[bot]]

Backport 11ee2b9c4233c2c8d28a15ff63a98a4dcc72dc7f from #10359.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.28%. Comparing base (e174759) to head (5e56975).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 1.8.latest #10361 +/- ## ============================================== - Coverage 88.32% 88.28% -0.04% ============================================== Files 181 181 Lines 22758 22758 ============================================== - Hits 20101 20093 -8 - Misses 2657 2665 +8 ``` | [Flag](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10361/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `85.46% <ø> (-0.16%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `62.80% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs#carryforward-flags-in-the-pull-request-comment) to find out more.

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