messages: migrate to new bs4 API

[emmanuel-ferdman]

What does this PR do, and why?

Small PR - resolves the bs4 deprecation warnings which you can see in the CI logs:

/home/runner/work/zulip-terminal/zulip-terminal/zulipterminal/ui_tools/messages.py:941: DeprecationWarning: Access to deprecated property nextSibling. (Replaced by next_sibling) -- Deprecated since version 4.0.0.

/home/runner/work/zulip-terminal/zulip-terminal/zulipterminal/ui_tools/messages.py:918: DeprecationWarning: Call to deprecated method findChildren. (Replaced by find_all) -- Deprecated since version 3.0.0.

/home/runner/work/zulip-terminal/zulip-terminal/zulipterminal/ui_tools/messages.py:940: DeprecationWarning: Call to deprecated method findAll. (Replaced by find_all) -- Deprecated since version 4.0.0.

/home/runner/work/zulip-terminal/zulip-terminal/zulipterminal/ui_tools/messages.py:941: DeprecationWarning: Access to deprecated property nextSibling. (Replaced by next_sibling) -- Deprecated since version 4.0.0.

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

[neiljp]

@emmanuel-ferdman Thanks for the PR 👍

I adjusted the commit text slightly and am merging now that CI passed :)