Upgrade to ubuntu 20.04

[EnricoMi]

Lets see how well things work with Ubuntu 20.04.

[github-actions]

Unit Test Results

     773 files   -      56       773 suites   - 56   8h 36m 6s ⏱️ - 39m 28s
     722 tests  -        8       675 ✔️  -     4       47 💤 +    4  0 ❌ ±0 
16 693 runs   - 1 050  11 776 ✔️  - 676  4 917 💤  - 358  0 ❌ ±0 

Results for commit 11d3bce. ± Comparison against base commit 07e4e24.

This pull request removes 8 tests.

test.parallel.test_adasum_tensorflow
test.parallel.test_keras
test.parallel.test_process_sets_multi_comm
test.parallel.test_process_sets_static
test.parallel.test_tensorflow
test.parallel.test_tensorflow2_keras
test.parallel.test_tensorflow_keras
test.parallel.test_xla

This pull request skips 4 tests.

test.parallel.test_mxnet2.MX2Tests ‑ test_gluon_trainer
test.parallel.test_mxnet2.MX2Tests ‑ test_gpu_required
test.parallel.test_mxnet2.MX2Tests ‑ test_horovod_allreduce_cpu_gpu_error
test.parallel.test_mxnet2.MX2Tests ‑ test_horovod_grouped_allreduce_cpu_gpu_error

♻️ This comment has been updated with latest results.

[github-actions]

Unit Test Results (with flaky tests)

     879 files   -    116       879 suites   - 116   9h 45m 34s ⏱️ - 13m 27s
     722 tests  -        8       673 ✔️  -        3       47 💤 +    4  2 ❌  - 1 
19 178 runs   - 1 592  13 431 ✔️  - 1 054  5 743 💤  - 491  4 ❌ +1 

For more details on these failures, see this check.

Results for commit 11d3bce. ± Comparison against base commit 07e4e24.

This pull request removes 8 tests.

test.parallel.test_adasum_tensorflow
test.parallel.test_keras
test.parallel.test_process_sets_multi_comm
test.parallel.test_process_sets_static
test.parallel.test_tensorflow
test.parallel.test_tensorflow2_keras
test.parallel.test_tensorflow_keras
test.parallel.test_xla

This pull request skips 4 tests.

test.parallel.test_mxnet2.MX2Tests ‑ test_gluon_trainer
test.parallel.test_mxnet2.MX2Tests ‑ test_gpu_required
test.parallel.test_mxnet2.MX2Tests ‑ test_horovod_allreduce_cpu_gpu_error
test.parallel.test_mxnet2.MX2Tests ‑ test_horovod_grouped_allreduce_cpu_gpu_error

♻️ This comment has been updated with latest results.