Add deprecation and future disabling of playstore builds warning to banner

[agnostic-apollo]

Sorry for the duplicate. I forgot to re-commit before force pushing after undoing the single commit.

@Grimler91 and @buttaface thanks for the suggestions. I have tried to incorporate them from both of you. Its slightly shorter and will show even if banner is disabled.

I can't really say "The termux apps will be removed from Play store", since that requires involvement from @fornwall, who hasn't been responding on emails/sms.

I have updated https://github.com/termux/termux-app#installation as well and added deprecation details.

Let me know of any further changes.

[Grimler91]

Looks good!

This:

.. so that new users
cannot install them and to **disable** them for
existing users with updates so that they do not
continue using outdated versions.

could be modified to make it clear who "they" are in the second part of the sentence. Perhaps:

.. so that new users
cannot install them and to **disable** them for
existing users with app updates to prevent the use 
of outdated app versions.

[agnostic-apollo]

@Grimler91 Done!

If someone else doesn't suggest changes in next few hours, I'll bump termux-tools version.

[RalfWerner]

Do it, even if I do not mean playstore users have an advantage of it.