CI: Upgrade codecov-action from v3 to v4.

[neiljp]

What does this PR do, and why?

Actions are upgrading to node 20, which is anticipated to occur via this major version bump of codecov-action.

Other actions were upgraded recently; this action provided a note in a CI run which recommends upgrading this action too:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: codecov/codecov-action@v3.
For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

(https://github.com/zulip/zulip-terminal/actions/runs/7838963350?pr=1468)

NOTE: This is currently paused over concern over the status of https://github.com/codecov/feedback/issues/112

zulip/zulip currently uses v3 of the action. However, it seems that v3 is unlikely to bump to node 20 due to a need to avoid breakage, and will become unsupported due to its use of node 16, except for v3.1.5. So possibly this could be addressed with Zulip via an organization secret?

Outstanding aspect(s)

• [ ] Concern over possible breakage? (see above)

External discussion & connections

• [ ] Discussed in #zulip-terminal in topic
• [ ] Fully fixes #
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] Adapting existing automated tests
• [ ] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [ ] It is a minimal coherent idea
• [ ] It has a commit summary following the documented style (title & body)
• [ ] It has a commit summary describing the motivation and reasoning for the change
• [ ] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit