net/url: JoinPath() drops trailing slashes [1.18 backport]

[gopherbot]

@ianlancetaylor requested issue #52074 to be considered for backport to the next 1.18 minor release.

@gopherbot Please open backport to 1.18

This function is new in 1.18 so we should keep it consistent with this change.

[ianlancetaylor]

This function is actually not in 1.18, so there is nothing to do.

[jeffwidman]

Can this be removed from the 1.18.1 milestone given that it's not relevant?

[ianlancetaylor]

Done.