Prevent update handling task pointers from being garbage collected, backport of #1328

[Kylmakalle]

Description

This is a backport of #1328 to mitigate possible issues like #628, #927. As in #1328, this PR preserves asyncio Task pointers until the task is completed to ensure Task will not be garbage collected and disappear. See recommendations in Python docs.

Important: Save a reference to the result of this function, to avoid a task disappearing mid-execution. The event loop only keeps weak references to tasks. A task that isn’t referenced elsewhere may get garbage collected at any time, even before it’s done. For reliable “fire-and-forget” background tasks, gather them in a collection

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Honestly, I don't have a v3-based bot with workload I have in v2. But want to admit once again, it's a pretty rare case and we're handling it as the docs says.

Test Configuration:

• Operating System: Macos, Linux
• Python version: 3.11.1

[github-actions]

✔️ Changelog found.

Thank you for adding a description of the changes

[Kylmakalle]

I've not seen any evidence of "Task was destroyed but it is pending" for v3 yet, but I assume it's due to a lower adoption rate than v2.

[codecov]

Codecov Report

Merging #1331 (cd3dc7c) into dev-3.x (6738255) will not change coverage.
Report is 1 commits behind head on dev-3.x.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##           dev-3.x     #1331   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          369       369           
  Lines         9429      9435    +6     
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+ Hits          9429      9435    +6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
aiogram/dispatcher/dispatcher.py	100.00% <100.00%> (ø)
aiogram/webhook/aiohttp_server.py	100.00% <100.00%> (ø)