Rename servlet-api-3_1 to match Gerrit master

Rename servlet-api-3_1 to match Gerrit master

JGit is built from source on Gerrit-CI: the Bazel build
dependencies need to be aligned. On Gerrit master the servlet-api-3_1
is renamed to servlet-api (see [1]): do the same on the JGit master
branch.

In addition, removing the version suffix from dependency rule makes
it much easier to upgrade. The actual reason the for the renaming it
in Gerrit core is upgrade attempt to servlet-api 4.0, done in [2].

[1] https://gerrit-review.googlesource.com/c/gerrit/+/238496
[2] https://gerrit-review.googlesource.com/c/gerrit/+/238383

Change-Id: I63c506da335deef54eb279879e897b427200797b
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>