"install" hook error caused by certifi library dropping python 3.5 supports

[Tianqi Xiao]

The telegraf charm is using requests python library when dealing with units with a public address (ex. percona-cluster). One of its dependencies, certifi library, has dropped python 3.5 support since version 2022.05.18. Consequently, the telegraf charm resulted in error state when running the "install" hook.

```
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install certifi requires Python '>=3.6' but the running Python is 3.5.2
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install Traceback (most recent call last):
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install File "/var/lib/juju/agents/unit-telegraf-0/charm/hooks/install", line 8, in <module>
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install basic.bootstrap_charm_deps()
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install File "lib/charms/layer/basic.py", line 226, in bootstrap_charm_deps
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install env=_get_subprocess_env())
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install File "/usr/lib/python3.5/subprocess.py", line 581, in check_call
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install raise CalledProcessError(retcode, cmd)
unit-telegraf-0: 18:30:25 WARNING unit.telegraf/0.install subprocess.CalledProcessError: Command '['/var/lib/juju/agents/unit-telegraf-0/.venv/bin/pip', 'install', '-U', '--force-reinstall', '--no-index', '--no-cache-dir', '-f', 'wheelhouse', 'certifi==2022.6.15', 'netaddr==0.7.19', 'defusedxml==0.5.0', 'urllib3==1.24.3', 'insights-core==3.0.279', 'tenacity==5.0.3', 'pyaml==21.10.1', 'msgpack==1.0.3', 'CacheControl==0.12.6', 'six==1.16.0', 'Jinja2==2.11.3', 'PyYAML==5.3.1', 'chardet==3.0.4', 'pbr==5.9.0', 'lockfile==0.12.2', 'charms.reactive==1.5.0', 'charmhelpers==1.2.1', 'idna==2.8', 'redis==3.3.11', 'requests==2.21.0']' returned non-zero exit status 1
unit-telegraf-0: 18:30:26 ERROR juju.worker.uniter.operation hook "install" (via explicit, bespoke hook script) failed: exit status 1
```

So we need to limit the certifi version to ensure the python 3.5 support.

[Tianqi Xiao (txiao)]

The final decision on handling dependency issues in legacy series (e.g. xenial) is:
- Drop legacy series support in main development track
- Create a new branch and channel to track legacy series