docs: update Ubuntu version reference in Quick Install

[Mannan-Ali]

This PR updates the Quick Install documentation to be version-agnostic, resolving the outdated Ubuntu 18.04 reference.

Resolves #751 (Originally reported in goharbor/harbor#23666)

[Vad1mo]

this should be filled agains the main branch not an old unsupported branch

[Mannan-Ali]

@Vad1mo , The 'Quick install' section isn't present in the main branch anymore and seems to only exist in the release-2.1.0 branch.
So do you want me to add the whole section again in main branch or should i leave it and close the issue as the original issue raisied here (goharbor/harbor#23666), refers to the particular 2.1.0 version??

This is how it looks in 2.1.0 version:

This is how it looks in main version:

[Vad1mo]

ok, so why you want to change something that is 5 y old and archived history?

[Mannan-Ali]

As the main issue raised here (goharbor/harbor#23666) was for version 2.1.0 thats why I asked in my raised issue should I make a pr or not, anyway if its not needed I will close the issue. @Vad1mo Thanks for the clarification.