Change the go version in Makefile to 1.18

[bharath-pf9]

Testing to see If the version is something that's causing pf9ctl not work properly.

Output that we get when testing with latest pf9ctl,

* Connection #0 to host pmkft-assets.s3.us-west-1.amazonaws.com left intact
/root/pf9/bin/pf9ctl_new: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /root/pf9/bin/pf9ctl_new)
/root/pf9/bin/pf9ctl_new: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /root/pf9/bin/pf9ctl_new)
16:14:37.694003794 :63 : ASSERT: Installation of Platform9 CLI Failed

With this change, pf9ctl install works fine.

Making an additional change to match regex with double digits e.g., to match '8.10'