Mention PHP extensions bcmath and gmp in INSTALL.md,
because openid requires one of them to be installed.

Resolves: #55635
Releases: 6.2
Change-Id: I6ab651d3cb863a570b82fc33231e746e897bee96
Reviewed-on: https://review.typo3.org/27309
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind