So you have Windows set to open .jar files with notepad, this is incorrect.
and as the file association with Java and Jar files is a bit special, you'll want to run JarFix once to fix it for you
(the alternative is a full reinstall of Java or tweaking the registry yourself)
Delete all the files mentioned in this block of the log (if they exist) and try again:
Splitter:
Input: C:\Users\Arslan\AppData\Roaming\.minecraft\versions\1.16.3\1.16.3.jar
Input: C:\Users\Arslan\AppData\Roaming\.minecraft\versions\1.16.3\1.16.3.jar
Slim: C:\Users\Arslan\AppData\Roaming\.minecraft\libraries\net\minecraft\client\1.16.3-20200911.084530\client-1.16.3-20200911.084530-slim.jar
Data: null
Extra: C:\Users\Arslan\AppData\Roaming\.minecraft\libraries\net\minecraft\client\1.16.3-20200911.084530\client-1.16.3-20200911.084530-extra.jar
Including data in extra
SRG: C:\Users\Arslan\AppData\Roaming\.minecraft\libraries\de\oceanlabs\mcp\mcp_config\1.16.3-20200911.084530\mcp_config-1.16.3-20200911.084530-mappings.txt
InputSha: 3d2cba3c97cb57dd3f2a048933cb32ef6abd4639
SrgSha: de180369283e7aafbb5694c4b8995a9c83fe0a95
We don't intentionally disable anything to do with legacy versions, we just don't fix them when things go wrong anymore.
Locking because 1.7.10 is old and it should have been allowed to die years ago.
Port forwarding is exposing a device on your network to the internet through a specific port.
This is something you'll have to look up on your own as it's different for every router.