README.md: Fix link to conan packages

Revert "CMake: Search for ssh2 instead of libssh2."

proxy: Return an error for invalid proxy URLs instead of crashing.

Test proxies specified by both host:port format in configuration
options, environment variables, and `http.proxy` configuration.

The common format for specifying proxy URLs is just 'host:port'. Handle
the common case.

Introduce a url parser that defaults to treating poorly specified URLs
as http URLs. For example: `localhost:8080` is treated as
`http://localhost:8080/` by the http-biased url parsing, instead of a
URL with a scheme `localhost` and a path of `8080`..

Refactor url parsing to simplify the state-passing (introducing a
struct) and add a path parser for future reusability.

ssh: fix known_hosts leak in _git_ssh_setup_conn

stransport: macOS: replace errSSLNetworkTimeout, with hard-coded value

- Constant only available in 10.13+, causing build failures for older macOS releases
Fixes: https://github.com/libgit2/libgit2/issues/6606

fix: Add missing include for oidarray.

Add a missing include for `git2/oidarray.h` so build doesn't fail on
using `git_oidarray` when using custom transports.

Fixes #6607

repository: make cleanup safe for re-use with grafts

We are allowed to call `git_repository__cleanup` multiple times, and this
happens e.g. in rugged if we want to free up resources before GC gets around to
them.

This means that we cannot leave dangling pointers in it, which we were doing
with the grafts. Fix this by setting the pointers to NULL after freeing the resources.

meta: update version numbers to v1.8

v1.7: release Kleine Raupe Nimmersatt

deps: update pcre to 8.45

#6576 git_diff_index_to_workdir reverse now loads untracked content

tests: add allocator with limited number of bytes

Fixes #6344: git_branch_move now renames the reflog instead of deleting.

Partial fix for #6532: insert-by-date order.

midx: do not try to look at every object in the index

pack: cast the number of objects to size_t

CMake: Search for ssh2 instead of libssh2.

midx: allow unknown chunk ids in multi-pack index files

docs: fix IRC server from freenode to libera

This commit update the contributing documents with the
new IRC server and remove the old freenode.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>