doc: update the version support matrix

- Retired rust_icu 3.0.
- Retired support for ICU 70 to 73.
- Added a notice that rust_icu compiles well under 76.0, given the new compile workflow.
- We skipped ICU 75, but I'm fairly confident it would have worked.

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        icu_version: [63, 72, 73, 74]
+        icu_version: [63, 73, 74]
     steps:
       - uses: actions/checkout@v2
       - name: 'Test ICU version ${{ matrix.icu_version }}'

README.md

@@ -121,11 +121,10 @@ The compatibility guarantee is as follows:
 2. Automated tests are executed for the ICU library version in use by the docs.rs
    system (so the documentation could be built).
 
-`rust_icu` version   | ICU 63.x | ICU 70.1 | ICU 71.1 | ICU 72.1 | ICU 73.1 | ICU 74.1 |
--------------------- | -------- | -------- | -------- | -------- | -------- | -------- |
-3.0                  |    ✅    |    ✅    |    ✅    |    ✅    |          |          |
-4.0                  |    ✅    |          |    ✅    |    ✅    |    ✅    |          |
-5.0                  |    ✅    |          |          |    ✅    |    ✅    |    ✅    |
+`rust_icu` version   | ICU 63.x | ICU 73.1 | ICU 74.1 | ICU 75.0 | ICU 76.0 |
+-------------------- | -------- | -------- | -------- | -------- | -------- |
+4.0                  |    ✅    |    ✅    |          |          |          |
+5.0                  |    ✅    |    ✅    |    ✅    |    X     |    ✅    |
 
 # Features