Fix obsoleted function warnings for newer Emacs

[syohex]

There is the following byte-compile warning on Emacs 29 or higher versions. This patch fixes it.

In go-guru--goto-byte-column:
go-guru.el:321:52: Warning: ‘point-at-bol’ is an obsolete function (as of
    29.1); use ‘line-beginning-position’ or ‘pos-bol’ instead.

[Hi-Angel]

@dominikh ping? I was just looking at fixing the warnings as well, with modern Emacs go-mode.el produces so many of them

[dominikh]

Deprecating an alias. Great.