Disable two libgit2 tests with libgit2 < 0.26.0 (#21872)

Without our patches libgit2-remote-push-NULL and libgit2-gitconfig-symlink,
which are not yet included in an upstream release, these tests fail.
Disable them so that builds using the system libgit2 can pass the tests.
(cherry picked from commit a204bfd)

test/libgit2.jl

@@ -99,7 +99,8 @@ end
     end
 end
 
-is_unix() && @testset "Default config with symlink" begin
+# See #21872 and #21636
+LibGit2.version() >= v"0.26.0" && is_unix() && @testset "Default config with symlink" begin
     with_libgit2_temp_home() do tmphome
         write(joinpath(tmphome, "real_gitconfig"), "[fake]\n\tproperty = BBB")
         symlink(joinpath(tmphome, "real_gitconfig"),
@@ -854,8 +855,10 @@ mktempdir() do dir
             end
             LibGit2.add!(our_repo, "file1")
             LibGit2.commit(our_repo, "add file1")
-            # we cannot yet locally push to non-bare repos
-            @test_throws LibGit2.GitError LibGit2.push(our_repo, remoteurl=up_path)
+            if LibGit2.version() >= v"0.26.0" # See #21872, #21639 and #21597
+                # we cannot yet locally push to non-bare repos
+                @test_throws LibGit2.GitError LibGit2.push(our_repo, remoteurl=up_path)
+            end
         finally
             close(our_repo)
             close(up_repo)