Unable to use build cache with latest `spack` version

I am trying to use the e4s build cache with spack:

> spack mirror add E4S https://cache.e4s.io
> spack buildcache keys -it                                                                      [238/391]
==> Fetching https://cache.e4s.io/build_cache/_pgp/25645FA2B218FE55B4EF649E4345F04B40005581.pub           
==> Fetching https://mirror.spack.io/bootstrap/github-actions/v0.4/build_cache/linux-centos7-x86_64-gcc-10.2.1-patchelf-0.16.1-p72zyan5wrzuabtmzq7isa5mzyh6ahdp.spec.json                                           
==> Fetching https://mirror.spack.io/bootstrap/github-actions/v0.4/build_cache/linux-centos7-x86_64/gcc-10
.2.1/patchelf-0.16.1/linux-centos7-x86_64-gcc-10.2.1-patchelf-0.16.1-p72zyan5wrzuabtmzq7isa5mzyh6ahdp.spac
k                                                                                                         
==> Installing "patchelf@=0.16.1%gcc@=10.2.1 ldflags="-static-libstdc++ -static-libgcc"  build_system=auto
tools arch=linux-centos7-x86_64" from a buildcache                                                        
==> Fetching https://mirror.spack.io/bootstrap/github-actions/v0.4/build_cache/linux-centos7-x86_64-gcc-10
.2.1-clingo-bootstrap-spack-cizcjj3tx4irs3uzoktrgybq73sz545f.spec.json                                    ==> Fetching https://mirror.spack.io/bootstrap/github-actions/v0.4/build_cache/linux-centos7-x86_64/gcc-10
.2.1/clingo-bootstrap-spack/linux-centos7-x86_64-gcc-10.2.1-clingo-bootstrap-spack-cizcjj3tx4irs3uzoktrgyb
q73sz545f.spack                                                                                           
==> Installing "clingo-bootstrap@=spack%gcc@=10.2.1~docs~ipo+python+static_libstdcpp build_type=Release arch=linux-centos7-x86_64" from a buildcache                                                                
gpg: key 4345F04B40005581: public key "University of Oregon - E4S" imported                               
gpg: Total number processed: 1                                                                            
gpg:               imported: 1                                                                            
gpg: inserting ownertrust of 6                  

However, installing packages fails with:

> spack install cuda@11.5.2 gcc@11.1.0 
==> Warning: you need a newer Spack version to read the buildcache index for the following mirror: 'https:
//cache.e4s.io'. The expected DB version is '6', but '7' was found.
# spack proceeds to install packages from source

spack v0.20.2 (Arch Linux)

The issue names no repository file, test, or entry point. Start by reproducing the build-cache warning with Spack v0.20.2 and the E4S mirror, then inspect the repository for related build-cache or Spack documentation. Done would require identifying and addressing the version incompatibility, with the resulting installation behavior documented or verified.