Fix setting `silence` of `warn_error_options` via `dbt_project.yaml` flags

[QMalcolm]

resolves #10160

Problem

Specifying silence of warn_error_options in dbt_project.yaml flags wasn't silencing warnings.

Solution

Ensure silence is used during instantiation of WarnErrorOptions from dbt_project.yaml

Checklist

• [X] I have read the contributing guide and understand what's expected of me
• [X] I have run this code in development and it appears to resolve the stated issue
• [X] This PR includes tests, or tests are not required/relevant for this PR
• [X] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• [X] This PR includes type annotations for new and modified functions

[codecov[bot]]

Codecov Report

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

Project coverage is 88.75%. Comparing base (d389ff1) to head (4cb35a1). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10359 +/- ## ========================================== - Coverage 88.78% 88.75% -0.04% ========================================== Files 180 180 Lines 22479 22486 +7 ========================================== - Hits 19958 19957 -1 - Misses 2521 2529 +8 ``` | [Flag](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10359/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/10359/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `86.01% <ø> (-0.07%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10359/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `62.19% <ø> (+0.01%)` | :arrow_up: | 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.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.