Update lambas for C++20

[RytoEX]

Description

Replace implicit lambda captures to be explicit for C++20 compliance while retaining C++17 compatibility.

Motivation and Context

Want obs-studio and its submodules to build with C++20 some day.

However, websocketpp, used by obs-websocket, is not C++20 compatible, so we will need to choose one of the following options:

1. Patch websocketpp.
2. Build obs-websocket as C++17 while the rest of the project builds as C++20.
3. Replace websocketpp.

That said, I wanted to at least push the required obs-websocket changes here for visibility/review.

How Has This Been Tested?

Tested OS(s): Windows 11

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

• Other Enhancement (anything not applicable to what is listed)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.

[PatTheMav]

Approving mainly because it makes explicit what happened implicitly before, memory (un)safety is unchanged.