release: set petsc v3.16.0 strings

Merge branch 'release'

Increase patchlevel to 3.15.5

PetscStack is on the stack now, removing the need to 'initialize' it so it can always be
used. The entire API has also been intern'ed since it is only internally used in select
instances.

Update gpu_roadmap.rst

Merge branch 'jwallwork23/metric-utils' into 'main'

Plex metric utils

See merge request petsc/petsc!4337

sh-compatible petscmpiexec

include valgrind suppression files into tarball

doc changes: add metric utils to developer docs

Update H2 package

Add VecBoundtoCPU and MatBoundToCpu queries

Merge branch 'rmills/feature-vecmean' into 'main'

Add VecMean() to calculate arithmetic mean of the elements of a vector.

See merge request petsc/petsc!4326

Implicit Runge-Kutta integrators

Add VecMean() to calculate arithmetic mean of the elements of a vector.

This is just a convenience wrapper around VecSum(), but having it is consistent with having MatGetColumnMeans().

Merge branch 'rmills/feature-kaij-update-AIJ-OAIJ-when-needed' into 'main'

Add lazy/just-in-time updating of internal data structures for MATMPIKAIJ matrices

See merge request petsc/petsc!4340

Use MatKAIJ_build_AIJ_OAIJ() to ensure AIJ, OAIJ are up to date throughout the KAIJ routines.

Thanks-to: Mr. Hong Zhang <hongzhang@anl.gov>

Updates to the core developers and emeritus developers:

Merge remote-tracking branch 'origin/release'

TAO updates in users manual

Merge branch 'psanan/docs-image-submodule-2' into 'release'

Move images to separate repository

See merge request petsc/petsc!4275

Docs: update ANL tech report number

Developer docs: remove rule about using "if (!rank)"

Merge remote-tracking branch 'origin/release'

Merge branch 'stefanozampini/cray-arm' into 'main'

Initial support on A64FX with Cray compilers

Closes #991

See merge request petsc/petsc!4312

Updates to the TAO acknowledgment based on updated core and emeritus developers.

Docs: use logo and favicon from image submodule