fix: add missing delete_after parameter

[Seltsamsel]

Summary

This fixes a typing error when doing something like

await ctx.defer()
await ctx.followup.send('something', ephemeral=True, delete_after=15)

error: No overload variant of "send" of "Webhook" matches argument types "str", "bool", "int"  [call-overload]
note: Possible overload variants:
note:     def send(self, content: str = ..., *, username: str = ..., avatar_url: Any = ..., tts: bool = ..., ephemeral: bool = ..., file: File = ..., files: List[File] = ..., embed: Embed = ..., embeds: List[Embed] = ..., allowed_mentions: AllowedMentions = ..., view: View = ..., thread: Snowflake = ..., thread_name: Optional[str] = ..., wait: Literal[True]) -> Coroutine[Any, Any, WebhookMessage]
note:     def send(self, content: str = ..., *, username: str = ..., avatar_url: Any = ..., tts: bool = ..., ephemeral: bool = ..., file: File = ..., files: List[File] = ..., embed: Embed = ..., embeds: List[Embed] = ..., allowed_mentions: AllowedMentions = ..., view: View = ..., thread: Snowflake = ..., thread_name: Optional[str] = ..., wait: Literal[False] = ...) -> Coroutine[Any, Any, None]

I guess when the delete_after parameter was added it was simply forgotten in the overloads.

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
• I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[Lulalaby]

Changelog

[JustaSqu1d]

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