doc: Fix accessibility bug in MSRV badge

The SVG badge showing the project's MSRV, `etc/msrv-badge.svg`, has
four occurrences of the MSRV in it. Three had the current intended
MSRV. But the top-level `svg` element's `aria-label` attribute
still had the value `rustc: 1.70.0+`, having not been updated the
most recent two times the badge SVG was adjusted for a new MSRV.
That would, at least potentially, result in screen readers and
possibly other software saying/outputting 1.70.0 instead of 1.75.0.

This changes the `aria-label` value to `rustc: 1.75.0+`, as
intended.

This also rephrases the comment in `msrv.yml` about updating the
badge when changing the MSRV, to mention the need to change "all
occurrences".

(We may want to add a CI check to make sure the badge is fully
consistent with the current MSRV, but that isn't attempted here.)

Author: EliahKagan

.github/workflows/msrv.yml

@@ -29,8 +29,9 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     env:
-      # dictated by `firefox` to support the `helix` editor, but now probably effectively be controlled by `jiff`, which also aligns with `regex`.
-      # IMPORTANT: adjust etc/msrv-badge.svg as well
+      # This is dictated by `firefox` to support the `helix` editor, but now probably effectively
+      # be controlled by `jiff`, which also aligns with `regex`.
+      # IMPORTANT: When adjusting, change all occurrences in `etc/msrv-badge.svg` as well.
       RUST_VERSION: 1.75.0
 
     steps: