Add upgrade test from latest release and from previous major version's latest release

[consideRatio]

I think it should be quite easy to add an upgrade test from the latest release, since we already have an upgrade test from the default branch on github.

the-littlest-jupyterhub/.github/integration-test.py

Lines 147 to 153 in adbd41d

	# Install TLJH from the default branch first to test upgrades
	if upgrade:
	run_container_command(
	test_name, "curl -L https://tljh.jupyter.org/bootstrap.py | python3 -"
	)
	run_container_command(test_name, f"python3 /srv/src/bootstrap.py {installer_args}")

z2jh has three upgrade tests:

1. from the latest dev release (for tljh this is equivalent to the main branch)
2. from the latest stable release (the previous release)
3. from the latest release of the previous major version

I figure it would make sense to have the same for tljh, where we currently have test 1 but is missing 2 and 3. (EDIT: see comment below, I think I'm incorrect - maybe we are testing 2 and not 1 currently?)

[consideRatio]

In #842 (comment) @manics wrote:

The installer should install the last tagged version.... if you want to be absolutely sure then do an install and check the version is 0.2.0 and not:

the-littlest-jupyterhub/setup.py

Line 5 in adbd41d

version="1.0.0.dev0",

Is the plan to add up upgrade test in a future PR before tagging 1.0.0?

I've checked, curl -L https://tljh.jupyter.org/bootstrap.py | sudo -E python3 - --admin admin installs 0.2.0, curl -L https://tljh.jupyter.org/bootstrap.py | sudo -E python3 - --admin admin --version main installs the main branch

---

Based on that, I think I was incorrect in the comment above about what upgrade test we currently do.