Fix byte-compile warnings

[syohex]

point-at-bol and point-at-eol are obsoleted since Emacs 29.1. Emacs 24.1 which is minimum supported version of yaml-mode supports line-beginning-position, line-end-position. https://github.com/emacs-mirror/emacs/blob/emacs-24.1/src/editfns.c#L774

yaml-mode.el:309:26: Warning: ‘point-at-eol’ is an obsolete function (as of
    29.1); use ‘line-end-position’ instead.
yaml-mode.el:310:19: Warning: ‘point-at-bol’ is an obsolete function (as of
    29.1); use ‘line-beginning-position’ instead.

In yaml-narrow-to-block-literal:
yaml-mode.el:429:17: Warning: ‘point-at-bol’ is an obsolete function (as of
    29.1); use ‘line-beginning-position’ instead.

[wasamasa]

Thank you.