Preview of a link is showing "Internet Explorer is not supported"

This issue has been migrated from [#15598](https://github.com/matrix-org/synapse/issues/15598).

---

### Description

URL previews seem to fetched with an outdated useragent string claiming IE. This can trigger warnings to update the browser in the URL preview instead of a helpful preview

### Steps to reproduce

Send a message with a link. (example https://delft.notubiz.nl/)
Matrix server will fetch a preview.
If this is a link to a website hat warns about outdated browsers, the preview shows the warning about outdated browser:
"Gemeente Delft Ga direct naar: Navigatie Let op: U gebruikt Internet Explorer. Vanaf 31 maart 2021 wordt Internet Explorer 11 niet meer ondersteund als browser door dit portaal. Wij adviseren u om gebruik te maken van Microsoft Edge, Google Chrome of Mozilla FireFox."

translation: "Municipality of Delft Go directly to: Navigation Please note: You are using Internet Explorer. From 31 March 2021, Internet Explorer 11 will no longer be supported as a browser by this portal. We advise you to use Microsoft Edge, Google Chrome or Mozilla FireFox."

What did you expect?

A preview of the site that the link points to
What happened instead?

The preview fetching passes a warning from the website about outdated browser, which is false.
Probable cause

I suspect that the useragent string used to fetch the preview is incorrect and outdated. It would be better if Element uses a useragent string that is modern and aligns with the OS on which it is running. (Element on Linux, not IE on Windows)
When testing with a useragent switcher tool (https://add0n.com/useragent-switcher.html) the website https://delft.notubiz.nl/ gives this warning when sending an IE useragent string, so the warning is useragent string dependent.

### Homeserver

matrix.org

### Synapse Version

"server_version":"1.83.0 (b=matrix-org-hotfixes,106fb7005d)","python_version":"3.8.12"

### Installation Method

I don't know

### Database

unknown

### Workers

I don't know

### Platform

unknown.

### Configuration

_No response_

### Relevant log output

```shell
Preview of https://delft.notubiz.nl/
"Gemeente Delft Ga direct naar: Navigatie Let op: U gebruikt Internet Explorer. Vanaf 31 maart 2021 wordt Internet Explorer 11 niet meer ondersteund als browser door dit portaal. Wij adviseren u om gebruik te maken van Microsoft"
```

### Anything else that would be useful to know?

Not my server.
Bug first reported in Element desktop: #961

Start by tracing Synapse's URL-preview fetching path and reproduce the issue with https://delft.notubiz.nl/. Inspect the user-agent used for the fetch and compare the resulting preview with a modern browser request. Done means the preview shows the site's content rather than an Internet Explorer warning.

Written by the indexing model from the issue text.