Update version for the next release (v0.15.0)

[meili-bot]

This PR is auto-generated.

The automated script updates the version of meilisearch-dart to a new version: "v0.15.0"

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (656bd85) 82.03% compared to head (3d58293) 82.03%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #355 +/- ## ======================================= Coverage 82.03% 82.03% ======================================= Files 57 57 Lines 1353 1353 ======================================= Hits 1110 1110 Misses 243 243 ``` | [Files Changed](https://app.codecov.io/gh/meilisearch/meilisearch-dart/pull/355?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=meilisearch) | Coverage Δ | | |---|---|---| | [lib/src/version.dart](https://app.codecov.io/gh/meilisearch/meilisearch-dart/pull/355?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=meilisearch#diff-bGliL3NyYy92ZXJzaW9uLmRhcnQ=) | `50.00% <ø> (ø)` | |

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[meili-bors[bot]]

Build succeeded:

• integration-tests (dart 3.0.0)
• linter-check
• pana