Update admin to lua-resty-session v4

- Upgrade [lua-resty-session](https://github.com/bungle/lua-resty-session) from 3.x to 4.x: https://github.com/NatLabRockies/api-umbrella/blob/1cb2bd548e7c1415deb3074fa9f899b99e5b9cd1/src/api-umbrella-git-1.rockspec#L20
- At the same time, upgrade [lua-resty-openidc](https://github.com/zmartzone/lua-resty-openidc) from 1.7.x to 1.8.x (since this is needed for lua-resty-session 4.x compatibility): https://github.com/NatLabRockies/api-umbrella/blob/1cb2bd548e7c1415deb3074fa9f899b99e5b9cd1/src/api-umbrella-git-1.rockspec#L19C4-L19C21

There is some in-progress work started on this in the `session-v4` branch: https://github.com/NatLabRockies/api-umbrella/compare/main...session-v4 Unfortunately, I forget where that really stands or what the remaining work was (it looks like at the very least the new `encrypt_lua_resty_session_cookie` implementation is incomplete). I think that work was maybe belatedly committed but actually predated lua-resty-openidc 1.8 coming out, which may have been the primary blocker. But now that both dependencies should be compatible with one another, I'd like to resurrect this work and get things upgraded.