fix: restore Redis as optional dependency

[faverl]

What

redis was removed from the list of required dependencies and restored as an optional dependency under the [redis] extra, the same behavior it had in version 1.6.2.

Reason

In version 1.6.3, redis>=7.4.0,<8.0.0 was added as an unconditional dependency along with its existing optional entry, causing it to always be installed with fastapi-mail, regardless of whether the Redis functionality was needed.

[sabuhish]

Hi, thanks for PR, totally agreed. But you still did not remove it from pyproject.toml file, it will be downloaded.
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[faverl]

Hi, thanks for the review!

Just to clarify the intent of this PR — Redis was already an optional dependency. The [project.optional-dependencies] entry has existed for a long time:

[project.optional-dependencies]
redis = ["redis"]

The issue is that in commit cbd97a4, this line was added to the main dependencies array:
"redis (>=7.4.0,<8.0.0)"

That broke the optional behavior — Redis started being installed by default, even those who didn't need it.

This PR is not introducing Redis as optional. It's restoring the previous, intentional behavior:

Removed redis (>=7.4.0,<8.0.0) from the main dependencies array.
Moved the version constraint into the existing optional entry, so it now reads:
redis = ["redis>=7.4.0,<8.0.0"]

Redis is still fully supported via:
pip install fastapi-mail[redis]