Summary:
inuse heap profiles of the api_service showed a large leak from a
alloc at `net/textproto.(*Reader).ReadMIMEHeader`. Some investigation revealed
that this could be due to some of the `gorilla` packages we used.

We used to depend on `gorilla/sessions@v1.1.3` which in turn
depended on `gorilla/context@v1.1.1`.
As per https://groups.google.com/g/gorilla-web/c/clJfCzenuWY/m/N_Xj9-5Lk6wJ
and https://github.com/gorilla/sessions/pull/23/files it seems like the
`gorilla/context` lib relied on explicit removal of request variables at the
end of the request lifetime.

As of `gorilla/sessions@v1.2.0` they switched over to the golang `context`
package and removed the dependency on `gorilla/context`. The golang `context`
lib no longer holds on to the same request variable and hence doesn't exhibit
the same leaky behavior.

Note, I believe the upgrade to `gorilla/handlers` isn't necessary to fix this
leak but it seemed like a good time to upgrade both gorilla deps.

Test Plan:
inuse heap profiles on a skaffold deploy of cloud no longer
seem to exhibit at leak from `ReadMIMEHeader`

Reviewers: zasgar, michelle, #third_party_approvers

Reviewed By: michelle, #third_party_approvers

JIRA Issues: PP-2740

Differential Revision: https://phab.corp.pixielabs.ai/D8636

GitOrigin-RevId: 97ce53644aeed0654d31ecf3f751c71e626aa79a