fix(deps): update version.slack to v1.39.3 (release/8.3)

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.slack.api:slack-app-backend	1.39.2 -> 1.39.3
com.slack.api:slack-api-model	1.39.2 -> 1.39.3
com.slack.api:slack-api-client	1.39.2 -> 1.39.3

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Release Notes

slackapi/java-slack-sdk (com.slack.api:slack-app-backend)

### [`v1.39.3`](https://togithub.com/slackapi/java-slack-sdk/releases/tag/v1.39.3): version 1.39.3 [Compare Source](https://togithub.com/slackapi/java-slack-sdk/compare/v1.39.2...v1.39.3) ### Changes - \[slack-api-client] [#​1316](https://togithub.com/slackapi/java-slack-sdk/issues/1316) Fix [#​1314](https://togithub.com/slackapi/java-slack-sdk/issues/1314) files.upload v2 method does not upload multiple files with full metadata - Thanks [@​oppokui](https://togithub.com/oppokui) [@​seratch](https://togithub.com/seratch) *** - All issues/pull requests: https://github.com/slackapi/java-slack-sdk/milestone/99?closed=1 - All changes: https://github.com/slackapi/java-slack-sdk/compare/v1.39.2...v1.39.3

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