fix go.mod replace for api by using older semantics

[tgross]

No description provided.

[shoenig]

For reference, the new behavior is described in https://golang.org/doc/go1.14#go-command

When the main module contains a top-level vendor directory and its go.mod file specifies go 1.14 or higher, the go command now defaults to -mod=vendor for operations that accept that flag. A new value for that flag, -mod=mod, causes the go command to instead load modules from the module cache (as when no vendor directory is present).
When -mod=vendor is set (explicitly or by default), the go command now verifies that the main module's vendor/modules.txt file is consistent with its go.mod file.

This is inconvenient for local development, where working on the api package requires explicitly setting -mod=mod, and totally breaks integration with IDEs like GoLand (e.g. https://youtrack.jetbrains.com/issue/GO-9428)

For now, setting go1.13 in the go.mod file alleviates the problem, as newer versions of the Go compiler respect the older build semantics.

It would be nice if the compiler would be more intelligent about submodules, especially in this case (e.g. golang/go#33789)

[github-actions]

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