web-api(docs): fix the sunset year for the files.upload method

[zimeg]

Summary

This PR fixes a mistake made in #1802 that was merged after previous approvals. Didn't expect an off-by-one error on years today, but here we are.

Reference: https://api.slack.com/changelog/2024-04-a-better-way-to-upload-files-is-here-to-stay

Requirements

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

[codecov[bot]]

Codecov Report

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

Project coverage is 81.63%. Comparing base (eca4d68) to head (fbb82e4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1805 +/- ## ======================================= Coverage 81.63% 81.63% ======================================= Files 35 35 Lines 7668 7668 Branches 316 316 ======================================= Hits 6260 6260 Misses 1396 1396 Partials 12 12 ``` | [Flag](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | Coverage Δ | | |---|---|---| | [cli-hooks](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.07% <ø> (ø)` | | | [cli-test](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `53.55% <ø> (ø)` | | | [oauth](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `76.51% <ø> (ø)` | | | [socket-mode](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `59.41% <ø> (ø)` | | | [web-api](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.48% <ø> (ø)` | | | [webhook](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/1805/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.20% <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=slackapi#carryforward-flags-in-the-pull-request-comment) to find out more.