docs: Fix error messages mentioning to mention upgrade_graph() instead of the nonexisting upgrade_version()

[louisaslett]

I tried to work with some old saved graphs and encountered the error message:

Error in warn_version(graph) : 
    This graph was created by a now unsupported old igraph version.
    Call upgrade_version() before using igraph functions on that object.

However, there is no upgrade_version() function exported by the igraph package, but there is an upgrade_graph() function which correctly upgrades the old objects. This pull requests just modifies the incorrect error message (and the test results which expect the error output).

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[krlmlr]

Thanks!

[krlmlr]

Good catch!

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This pull request failed to merge: PR cannot be automatically rebased, please rebase manually to continue. Remove the blocked label to re-queue.

Additional debug info: Failed to rebase this PR onto the latest changes from the base branch. You will probably need to rebase this PR manually and resolve conflicts).