Minimally update 'cart' terminology to 'gift'

[zweatshirt]

Description

NOTE:

• I currently can't access Give site with my email, whether it be production or staging. I'm working with the Service Desk to get this resolved. Because this is such a simple PR, I've opened it despite being able to test behavior on staging.

Jira ticket

• These are only partial changes related to the ticket, as there was an email stating this is all that is wanted to be changed for now.
• I do have a more extensive draft PR for when the time comes: EP-2612 - [Partial changes] Update cart header and empty cart message for clarity #1253

Testing

• Check that Cart terminology has been changed in accordance with the email discussion.

Checklist:

• I have given my PR a title with the format "EP-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels (Add the label "On Staging" to get the branch automatically merged into staging)
• I have requested a review from another person on the project
• I have checked that stage-branch-merger successfully merged my branch to staging or manually merged it myself
• I have tested my changes in staging for regular checkout
• I have tested my changes in staging for branded checkout

[stage-branch-merger]

I see you added the "On Staging" label, I'll get this merged to the staging branch!

[dr-bizz]

Thank you for doing this. This is working great. Let's push this up this morning as we have the code freeze on or after Friday.