docs/trans/ja_JP/howto: Don't mention specific kernel versions

docs/trans/ja_JP/howto: Don't mention specific kernel versions

    This change is based on commit d2b008f134b7
    ("Documentation/process/howto: Update for 4.x -> 5.x versioning").

    Replace "4.x kernel version" with generic term such as "mainline tree"

    Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
    ---
    V2: Reformatted commit log message (Yokosawa-san)
    V3: Updated some expressions (Shibata-san)
    - added "version number" in mainline tree section
    - updated from "stable kernel" to "stable tree"
    V4: Added reviewed by tag and removed extra characters (Yokosawa-san)
  　V5: Removed an extra character (Shibata-san)

Signed-off-by: Kosuke Fujimoto <fujimotokosuke0@gmail.com>
Acked-by: Tsugikazu Shibata <shibata@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220501102256.6379-1-fujimotokosuke0@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>