fix: base_url() removes trailing slash in baseURL

[kenjis]

Description
Fixes #7184

base_url() without arguments returns http://localhost:8080, not http://localhost:8080/.
The baseURL should have trailing slash, and there is no reason to remove it.
CI3's base_url() returns the URL with trailing slash.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[samsonasik]

Upgrade note seems need to be updated in case user use base_url() with their own tweak to add / after

[MGatner]

I don't recall why this was as such but please do indicate it in the changelogs as this could cause a mess for people, especially in tests.

[kenjis]

Added the docs.