[2.x] chore(core): improve a11y of avatar with no avatarUrl

[DavideIadeluca]

2.x equivalent to #4246

Fixes #0000

Changes proposed in this pull request:

• Removes the unnecessary loading="lazy" attribute for avatars with no avatarUrl
• Sets the aria role to img and adds an aria-label

Reviewers should focus on:

Screenshot
Before this Change:

<span class="Avatar Post-avatar" loading="lazy" style="--avatar-bg: #afa0e5;">M</span>

After this change:

<span class="Avatar Post-avatar" role="img" aria-label="moderator" style="--avatar-bg: #afa0e5;">M</span>

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)