Also make the `Decrypt` method private.

As far as I can tell, the only motivation for exposing the `SetBytes`
and `Reset` methods is to allow `CBoot::SetupWiiMemory` to use the same
`SettingsHandler` instance to read settings data and then write it back.
It seems cleaner to just use two separate instances, and require a given
`SettingsHandler` instance to be used for either writing data to a
buffer or reading data from a buffer, but not both.

A natural next step is to split the `SettingsHandler` class into two
classes, one for writing data and one for reading data. I've deferred
that change for a future PR.

We don't need to enforce the use of std::string instances with
AddSetting(). We can accept views and only construct one string,
rather than three temporaries.

Same behavior, way less verbose code.

fmt::localtime is also less awkward to use compared to std::localtime.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

https://bugs.dolphin-emu.org/issues/12019, take two.