config: use --value=<pattern> consistently

config: use --value=<pattern> consistently

This option was introduced in a series of commits from fe3ccc7aab (Merge
branch 'ps/config-subcommands', 2024-05-15).  But two styles were used
for the value provided to the option:

1. Synopsis: `--value=<value>`
2. Deprecated Modes: `--value=<pattern>`

(2) is also used in the synopsis on the command.

Use (2) consistently throughout since it’s a pattern in the general
case (`value` sounds more generic).

Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>