build: Update lower bound on tensorflow to v2.6.5

[matthewfeickert]

Description

Amends PR #1869. Following Discussion #1871, bump tensorflow lower bound to the oldest supported version by the TensorFlow project following tensorflow/tensorflow#56077.

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* Update lower bound on tensorflow to v2.6.5 as it is the "oldest" SemVer release
that is not broken on install by protobuf v4.21.0 ABI incompatibility.
   - c.f. https://github.com/tensorflow/tensorflow/issues/56077
   - Amend PR #1869
* Update tests/constraints.txt to use tensorflow==2.6.5.

[codecov]

Codecov Report

Merging #1874 (63c89c8) into master (ea3249a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1874   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files          68       68           
  Lines        4322     4322           
  Branches      726      726           
=======================================
  Hits         4243     4243           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.39% <ø> (ø)
doctest	60.73% <ø> (ø)
unittests-3.10	96.04% <ø> (ø)
unittests-3.7	96.02% <ø> (ø)

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[matthewfeickert]

Thomas's advice in PR #1869 combined with @henryiii's comment #1871 (comment)

Anyway, on the other side, I'd generally argue that you should support only supported versions. It's a lot of extra technical debt to support unsupported versions of anything, and convincing a user to upgrade is likely easier than maintaining old support, and crippling the new features and optimizations you could otherwise add. Someone can always select an older PyHF if they are on an older tensorflow. Someone okay with an old tensorflow / python / etc should be okay with an older pyhf.

seem convincing to me to make this PR.