Add Meson to the apt install dependency list and remove the Regression tests Section from the readme

[ErikGaida]

1. The installation instructions are outdated and currently lead to a PEP 668 violation when installing Meson via system-wide pip:

erik@FAME-1000658274-L14:~/rt-mbs-function$ sudo python3 -m pip install --upgrade meson
error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.
    
    If you wish to install a non-Debian-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
    sure you have python3-full installed.
    
    If you wish to install a non-Debian packaged Python application,
    it may be easiest to use pipx install xyz, which will manage a
    virtual environment for you. Make sure you have pipx installed.
    
    See /usr/share/doc/python3.12/README.venv for more information.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.

On current Debian/Ubuntu systems, this fails with an externally-managed-environment error because Python packages should not be installed system-wide using pip.

This change adds Meson to the apt dependency list and removes the system-wide pip install step.

2. Remove non-existent regression tests section

The README previously documented regression tests that do not exist. This section
has been removed.

[ErikGaida]

I added .cache/ to .gitignore because it contained a large number of temporary files.

[ErikGaida]

@davidjwbbc, I noticed that the README refers to a regression test:

cd ~/rt-mbs-function
meson test -C build regression

However, this test is not currently registered in the project. Running the command results in:

erik@FAME-1000658274-L14:~/rt-mbs-function/build$ cd ~/rt-mbs-function
meson test -C build regression

ERROR: *:regression test name does not match any test

When i list every test:

erik@FAME-1000658274-L14:~/rt-mbs-function$ meson test -C build --list
unit - open5gs:core
unit - open5gs:crypt
app - open5gs:sctp
unit - open5gs:unit
5gc - open5gs:registration
5gc - open5gs:vonr
5gc - open5gs:slice
epc - open5gs:attach
epc - open5gs:volte
epc - open5gs:csfb
app - open5gs:310014
app - open5gs:handover
epc - open5gs:non3gpp
5gc - open5gs:transfer
rt-common-shared:librtsdp-unit-tests
app - rt-5gc-service-consumers:bsf-service-consumer
unit - rt-5gc-service-consumers:mb-smf-sc-unit-tests

[dsilhavy]

@ErikGaida Please remove the command for the regression test from the Readme:

cd ~/rt-mbs-function
meson test -C build regression

[ErikGaida]

@dsilhavy, I have addressed your requested change by removing the regression test command from the README. I have also rebased this branch onto development to keep it up to date. I believe this PR is now ready to be merged.