gnu, doc, tests: Use ‘bootloader-configuration’ everywhere.

* doc/guix.texi (Proceeding with the Installation): Replace the old-style
‘grub-configuration’ with the newer ‘bootloader-configuration’ syntax.
* gnu/system/examples/vm-image.tmpl: Likewise.
* gnu/system/install.scm (installation-os): Likewise.
* gnu/tests.scm (%simple-os): Likewise.
* gnu/tests/install.scm (%minimal-os, %minimal-os-on-vda, %separate-home-os)
(%separate-store-os, %raid-root-os, %encrypted-root-os, %btrfs-root-os):
Likewise.
* gnu/tests/nfs.scm (%base-os): Likewise.
* tests/guix-system.scm (OS_BASE, make_user_config): Likewise.
* tests/system.scm (%os, %os-with-mapped-device): Likewise.