Workflows: Don't use jsdelivr mirror: not up-to-date, unnecessary and problematic

* Not up-to-date

jsdelivr takes some days or weeks to get latest erlang versions, compare
  https://builds.hex.pm/builds/otp/arm64/ubuntu-24.04/builds.txt
  https://cdn.jsdelivr.net/hex/builds/otp/arm64/ubuntu-24.04/builds.txt

* Unnecessary

As builds.hex.pm is already geodistributed, there's no need to add mirror
Reference: https://www.hex.pm/docs/mirrors

* Problematic in parallel jobs

The ci.yml workflow is split in separate jobs that run in parallel,
and all of them must use the exact same erlang version.
When using mirrors, different erlang versions may be used...
and then the compiled files require more recompilation, which fail
  ===> Compiling /home/runner/work/ejabberd/ejabberd/c_src/epam.c
  ===> /home/runner/work/ejabberd/ejabberd/c_src/epam.c:18:10: fatal error: security/pam_appl.h: No such file or directory
   18 | #include <security/pam_appl.h>
      |          ^~~~~~~~~~~~~~~~~~~~~

Instead of removing mirrors, an alternative solution would be
to specify exact OTP version: "28.3.2", but then we would need to update
all those strings in ci.yml for every erlang release.

.github/workflows/ci.yml

@@ -36,9 +36,6 @@ jobs:
     - uses: erlef/setup-beam@v1
       with:
         otp-version: ${{ matrix.otp }}
-        hexpm-mirrors: |
-          https://cdn.jsdelivr.net/hex
-          https://builds.hex.pm
 
     - uses: awalsh128/cache-apt-pkgs-action@latest
       with:
@@ -85,9 +82,6 @@ jobs:
     - uses: erlef/setup-beam@v1
       with:
         otp-version: ${{ matrix.otp }}
-        hexpm-mirrors: |
-          https://cdn.jsdelivr.net/hex
-          https://builds.hex.pm
 
     - uses: actions/download-artifact@v7
       with:
@@ -132,9 +126,6 @@ jobs:
     - uses: erlef/setup-beam@v1
       with:
         otp-version: ${{ matrix.otp }}
-        hexpm-mirrors: |
-          https://cdn.jsdelivr.net/hex
-          https://builds.hex.pm
 
     - uses: awalsh128/cache-apt-pkgs-action@latest
       if: matrix.otp < '28'
@@ -220,9 +211,6 @@ jobs:
     - uses: erlef/setup-beam@v1
       with:
         otp-version: ${{ matrix.otp }}
-        hexpm-mirrors: |
-          https://cdn.jsdelivr.net/hex
-          https://builds.hex.pm
 
     - uses: actions/download-artifact@v7
       with:
@@ -321,9 +309,6 @@ jobs:
     - uses: erlef/setup-beam@v1
       with:
         otp-version: ${{ matrix.otp }}
-        hexpm-mirrors: |
-          https://cdn.jsdelivr.net/hex
-          https://builds.hex.pm
 
     - uses: actions/download-artifact@v7
       with:

.github/workflows/weekly.yml

@@ -21,9 +21,6 @@ jobs:
     - uses: erlef/setup-beam@v1
       with:
         otp-version: ${{ matrix.otp }}
-        hexpm-mirrors: |
-          https://builds.hex.pm
-      ## https://cdn.jsdelivr.net/hex # Does not include Erlang 29 yet
 
     - uses: awalsh128/cache-apt-pkgs-action@latest
       with: