Update gosaml2 so it supports AES256GCM

[probakowski]

This change updates gosaml2 to our fork with added support for AES256GCM and AES192GCM for encrypted assertions.
I also run go mod tidy to cleanup go.mod file

Closes #10909

Note to reviewers: I'd ask for review of 2 things, this change and gravitational/gosaml2#4.
I moved master in fork to the commit we used before in go.mod (gravitational/gosaml2@757d23f) and started from there, you can check history at https://github.com/gravitational/gosaml2/commits/master

[Tener]

The change looks fine on its own, but I have questions:

• why do we need support for AES256GCM and AES192GCM? This PR "closes Stale DynamoDB Auth Service Items #10019", but it is already closed and doesn't mention either of the two...?
• our fork of gosaml2 is behind several commits from the original. perhaps we want to move it to the tip of the main branch?

[zmb3]

The change looks fine on its own, but I have questions:

• why do we need support for AES256GCM and AES192GCM? This PR "closes Stale DynamoDB Auth Service Items #10019", but it is already closed and doesn't mention either of the two...?
• our fork of gosaml2 is behind several commits from the original. perhaps we want to move it to the tip of the main branch?

@Tener that was a typo. This closes #10909

[Tener]

Thanks to @zmb3 for clarification regarding the original issue.

Nit: as mentioned, perhaps worth moving our fork to include latest additions to https://github.com/russellhaering/gosaml2.

Other than that, LGTM.

[zmb3]

LGTM.

If we want to update gosaml2 to the latest upstream commit, let's do that as a separate PR in case something breaks and we need to roll it back.

[probakowski]

@Tener @zmb3 thanks for review!
I'll merge it as is and test updating fork to upstream in separate PR.