ci: fix multi-version matrix failures on Perl < 5.24

The cpm dependency manager now requires Perl 5.24+, breaking the
Install Dependencies step on all older Perl containers (5.8-5.22).

Fix by removing the Install Dependencies step from the matrix job
entirely — all runtime and test dependencies (XSLoader, Carp,
Test::More) are core Perl modules. Also remove AUTHOR_TESTING and
RELEASE_TESTING env vars since author tests (Test::Pod,
Test::Pod::Coverage) are already covered by the ubuntu and disttest
jobs.

Author: toddr-bot

.github/workflows/testsuite.yml

@@ -75,9 +75,7 @@ jobs:
 
     env:
       PERL_USE_UNSAFE_INC: 0
-      AUTHOR_TESTING: 1
       AUTOMATED_TESTING: 1
-      RELEASE_TESTING: 1
 
     strategy:
       fail-fast: false
@@ -91,12 +89,6 @@ jobs:
       - uses: actions/checkout@v4
       - name: perl -V
         run: perl -V
-      - name: Install Dependencies
-        uses: perl-actions/install-with-cpm@v1
-        with:
-          sudo: false
-          cpanfile: "cpanfile"
-          args: "--with-develop"
       - run: perl Makefile.PL
       - run: make
       - run: make test