Restrict bokeh=3 support

[gjoseph92]

See discussion in #7327

• reverts YAML changes from Allow bokeh=3  #5648
• adds < 3 back into error messages
• adds an explicit check that the bokeh version is < 3, and does not show the dashboard otherwise. This is new. I would rather not show the dashboard at all and tell users why than silently show something misleading.
• adds tests for dashboard behavior with different bokeh versions. I couldn't find any; maybe there are some and I just didn't know where they were?

• Tests added / passed
• Passes pre-commit run --all-files

[gjoseph92]

Hmm clearly the mocking I added doesn't work when other tests have run before it.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       15 files  ±  0         15 suites  ±0   6h 46m 13s ⏱️ + 9m 14s
  3 224 tests +  3    3 138 ✔️ +  1    84 💤 +1  2 ❌ +1 
23 839 runs  +22  22 922 ✔️ +14  915 💤 +7  2 ❌ +1 

For more details on these failures, see this check.

Results for commit 5e57470. ± Comparison against base commit 69b74f9.

♻️ This comment has been updated with latest results.

[jrbourbeau]

Thanks @gjoseph92

[jrbourbeau]

Thanks for taking the time to add this test coverage and figure out the weirdness that was happening with these mocks not originally working as expected (when the entire test suite was run). It's good to see things working, but I will say the logic here looks a bit brittle and it would be good to simplify if possible. Let's include things here as they are, since they work, and will help us get the release out

[gjoseph92]

Agreed, it's highly brittle (as evidenced by it working the first time but failing when run in CI). Since all this logic happens at import time, brittle mocks are kind of the only option—we'd probably want to refactor things into a function, so patching it is much more predictable.