RTC: Replace payload name AV1X with AV1 for WebRTC. (#2760)

[Huachao]

Fix the WebRTC "no found valid AV1 payload type" issue

[codecov-commenter]

Codecov Report

Merging #2784 (294ae61) into 4.0release (1a4249d) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@              Coverage Diff               @@
##           4.0release    #2784      +/-   ##
==============================================
- Coverage       60.32%   60.31%   -0.01%     
==============================================
  Files             121      121              
  Lines           50972    50978       +6     
==============================================
  Hits            30747    30747              
- Misses          20225    20231       +6     

| Impacted Files | Coverage Δ | |'

Translated to English while maintaining the markdown structure:

'| Impacted Files | Coverage Δ | |
|---|---|---|
| trunk/src/app/srs_app_rtc_conn.cpp | 10.29% <0.00%> (-0.04%) | ⬇️ |

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Translated to English while maintaining the markdown structure:

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TRANS_BY_GPT3

[Huachao]

Ping @winlinvip @chen-guanghua @xiaozhihong , please help to review.

[winlinvip]

Thank you very much. 👍