Fix stable/train gate

Two things:

* Add attrs to lower-constraints

  (cherry picked from commit ee12a11e708fce6c982a85b58cd2c3899f13479e)

* Drop tempest and grenade testing

  Recently, these jobs started failing while installing tempest. The
  trouble seems to be:

  * tempest 26.1.0 depends on paramiko>=2.7.0
  * paramiko 3.0.0 is selected which in turn depends on
    cryptography >= 3.3 and bcrypt >= 3.2
  * cryptography 39.0.0 and bcrypt 4.0.1 are selected, neither of which
    get installed from binary wheels and both of which require a Rust
    toolchain to compile

  For some unknown reason, this used to be OK-ish -- the stable/train
  upper-constraints file would be respected, paramiko==2.6.0 would be
  installed with cryptography==2.8 and bcrypt==3.1.7, and pip would
  simply warn

    ERROR: tempest 26.1.0 has requirement paramiko>=2.7.0, but you'll
    have paramiko 2.6.0 which is incompatible.

  even though everything would still work. It's not at all clear how or
  why this stopped working -- there haven't been any recent changes to
  tempest's requirements or the stable/train constraints file, and the
  version of pip is the same. Given the age of the branch and its
  extended-maintenance state, dropping the testing seems best.

Change-Id: I61750a1083a1c97a6222ec9040f90980ee73acc8

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