Fix bridge/slack build breakage from slack-go/slack v0.23.1 API changes

[johnnyxmas]

Summary

Several fields the Slack bridge relied on moved or were renamed in the slack-go/slack dependency (pulled in via the recent dependabot bump), breaking compilation:

• slack.Msg field casing changed: TimeStamp -> Timestamp, ThreadTimeStamp -> ThreadTimestamp.
• slackevents.MessageEvent no longer carries its own top-level Attachments/Files fields. MessageEvent's custom UnmarshalJSON now always populates .Message (synthesized from the top-level JSON for regular messages, or the real nested object for message_changed), so all attachment/file handling now reads from ev.Message instead. This also makes the old top-level "legacy/RTM attachments blocks" callback-ID check in skipMessageEvent redundant with the existing ev.Message.Attachments check, so it's removed rather than duplicated.
• filesCached/fileCached/handleDownloadFile now take slack.File instead of slackevents.File, since file data now only exists as slack.Msg.Files ([]slack.File); the two File types are structurally different and not interchangeable.
• slack.FileUploadParameters was renamed to UploadFileParameters as part of the files.upload -> files.upload.v2 migration, and now takes a single Channel string (not Channels []string) and requires FileSize. The returned FileSummary no longer carries Shares, so uploadFile now does a follow-up GetFileInfo call to resolve the message id/thread timestamp of the uploaded file's share.

Test plan

• bridge/slack, bridge/config, bridge/irc all build clean
• go vet clean
• Existing bridge/slack tests pass
• Full binary build (previously failing) now succeeds and runs